In every shiur I've heard, Gemarah or otherwise, when referring to Maimonides, the one giving the shiur says "The Rambam". But when speaking of Rashi, I've only heard someone say "Rashi". How did this distinction come about? Why is it that people say "The Rambam says x, but Rashi says y"?
(Same is true for the Ramban, the Meiri, the Mordechai, the Rashbam, the Rosh etc. vs. Rashi, Rabbeinu Tam, Rabbeinu Yona, Tosafos)
This is pure conjecture (but I think any answer to this question will be conjecture) but perhaps the difference between Rashi/Tosafos and all other rishonim (not counting rishonim who are called "Rabbeinu" as there it would be quite strange to say "the Rabbeinu") lies in how people were studying.
Imagine they are studying the Talmud. Someone wants to bring up R. Solomon Ben Aderet. He can say "the Rashba says" and quote the Rashba and everything will be fine. The same goes for other rishonim. However, when studying the Talmud, Rashi and Tosafos are not separate from the Talmud like other rishonim are. The Talmudic page is Talmud, Rashi, and Tosafos. Now if someone wants to refer to Rashi or Tosafos generally (in the same way as the earlier example referenced R. Solomon Ben Aderet) there is perhaps potential for an issue. If someone says "the Rashi" or the "the Tosafos" it may seem as if he is referring to a specific Rashi or Tosafos on the page. Thus, leaving out the word "the" indicates that you are referring to Rashi and Tosafos in general and not to a specific text on the page. (Indeed, when they are specifying a specific Rashi or Tosafos text they will use the word "the", e.g. "the second-to-last Tosafos asks this" or "the Rashi we just read".) This issue does not exist for the other rishonim because the other rishonim are not part of the Talmudic page.
(Note that I think there is a sociological element here as well; it seems to me that it is really the "yeshivish" groups that refer to other rishonim with the word "the", whereas academics and those identifying with intellectual Modern Orthodoxy tend to not use "the" for anyone. For example, if you read the Torah U'madda Journal, or Tradition, or similar publications, I think you will find that they primarily write "Rambam" and not "the Rambam".)
Shout out to @Efraim and @AlBerko for their sevaros. I was mechavein to them in my analysis.
I'm going to post an unpopular answer and say that it simply developed this way FOR NO REAL THOUGHT OUT REASON, and in fact, there are exceptions both ways, found from the times of the Rishonim until now. Adding "the" before the title of a name may be a way of referring to something written (such as a book), or that it originally stood for Harav X.
Here are some examples of other Rishonim referring to the Rambam as "Rambam" rather than Harambam:
Many more examples can be found from various religious authorities over time, and the same is true (although to a lesser extent) to referring to Rashi as Harashi. A simple (Bar Ilan) database search confirms this.
Terrific question and I might have a hint.
The letter ר in Hebrew Rabbis titles might stand for two options: either רב or רבי (also רבינו). Although רב accepts the definitive article הרב, the other does not.
When referring to Rabbis, if the "original" acronym stood for Rabbi like Rabbi Shlomo Yitzchaki, it could not accept the article, whereas the "original" acronym stood for Rav, it could accept the article. THis is similar (somehow) to the difference between the Tanoim (Rabbi Akiva) and the Amoroyim (Rav Ashi). I would say it is the difference between undisputable and disputable.
The keyword here is "original". It is clear, that the use of those acronyms is traditional, not institutional. So seemingly, if a Rabbi was well respected, he might be called Harav (the level of the Amoroyim), and therefore הרמב"ם, הריטב"א, הרן etc. If he was perceived on a "higher" paternal level, he might be called Rabbi, like Rashi that was accepted unanimously.
Correct me if I'm wrong, but it seems that in English it is impossible to use the definitive article with Rabbis acronyms in any way, because in English, unlike ה in Hebrew, "the" can not be used with a title, like The Rabbi Berkowitz. So we can say הרמב"ם in Hebrew meaning הרב משה בן מימון, but in English it is wrong to say The Rambam, as it would mean The Rabbi Moyshe Ben Maymon.
The usage of the article when referring to the name of the book a Rabbi is known for, like החפץ חיים, הט"ז etc is actually acceptable. Like "the [book of] Chofetz Chaim"
Nb: As a native Modern Hebrew speaker, I do see a "very large degree of freedom" in the usage of the definitive article in the Oral Torah (so to speak, if not explicit mistakes), from the Mishna, through the Talmud and to all of the commentators and interpreters till today. So a lot of differences in the spelling of the names can be explained simply by inaccuracies and inconsistencies of the authors.
Firstly, the question imply that (in English) the word the is not used by Rashi, but any terms we have are really carried over from Hebrew. In Hebrew, Rashi was often referred to and is still by some as רשי הקדוש and that would be the only way to say that properly in Hebrew. Thus, were that carried over in a widespread fashion, he would have been indeed called The Holy Rashi or The Saintly Rashi, while including the word the. So perhaps there is an additional question my he and others were not as specificity referred to like that. Another answer could be in essence of what the translation comes to be and perhaps the more common knowledge of Rashi’s abbreviated name. Because neither would make sense with the word the in the front. The our Rabbi Moshe Ben Maimon The our/my Rabbi Shlomo Yitzchaki
But your point is still present in Wikipedia. In the Hebrew Wikipedia page for Rambam it starts his biography:
הרמב"ם נולד בקורדובה, שבאל-אנדלוס
By Rashi it starts:
רש"י נולד בעיר טרואה
It is important to note that this applies both by English (taken from Hebrew) and by Hebrew because these languages contain the word the and therefore can employ its use.
This is not present in other languages which do not employ the word the such as Russian and Chinese. (And other Slavic and Asian languages)
For example, copied from Rambam’s Wikipedia page in Russian:
Рамбам очень быстро стал играть важную роль...
Rambam (no the word existing or present) very quickly began to play an important role...
And by Rashi’s in Russian:
Раши родился в городе Труа,
Rashi (no the) was born in Truya (Troyes),
You are correct. Saying "The Rambam" isn't proper grammar (It's litterally saying "The Rabbi Moshe Ben Maimon". It's just not correct). The same is true with (the) Meiri, Mordechai, Rashbam, Rosh, etc. In terms of the Baalei HaTosafos, it is grammatically incorrect to say "Tosafos says", as "Tosafos" was a school of thought with many people subscribing to it, with their collected writings resulting in the "Tosafos" text we have on our Talmud pages. Since "Tosafos" is not actually a person, it is gramattically correct to say "The Baalei HaTosafos say/write" as opposed to "Tosafos says". If you want to say "Tosafos ___", you can say "The Tosafos text adds".
