Likely because there were multiple Hebrew-based dialects among the various Semitic people, including lingual differences between the different tribes of Israel.
The Moabites, for example, had a Hebrew dialect that was only slightly different from Hebrew (source). One famous example of this is the Meisha Stele, written either in the Phoenician script or ancient Hebrew, which are two similar alphabets and the words themselves are a form of Hebrew.
In Tanach we see that many people from Am Yisrael had names that were also used by the Edomites. Here's a list from Da'at Mikra:
However, Edomites also had words we don't recognize, such as the name of their chief deity, Qos (קוס) which brought upon names such as Qos-Gabar.
Rabbi Ahron Marcus in Barzilai explains that because different tribes bordered different non-Jewish nations, over time, their Hebrew evolved in different directions (he wrote a lot on the subject; the entire book is dedicated to the development of the Hebrew language. The following is only a small part of what he has to say on the matter of Hebraic dialects):
"With the building of the Temple, many architectural terms were added to the Hebrew language, influenced by Syrian architects and builders. These terms we learn from Kings I and Yechezkel, where they appear, hundreds of years later, in style differences and names and lingual varieties that we have yet completely interpret. Moreover, commerce ties with India, Arabia, Southern Africa and Egypt were developed. Due to the split between the two kingdoms, Judea and Israel, a situation came about that one bordered the nations that spoke a southern Semitic dialect and the other had neighbors who spoke a northern Semitic dialect...Yonah ben Amitai, from Gat Hachefer...his city of birth, Gat Chefer, was in the Tribe of Zevulun, not far from Tzidon, and was surrounded by the Phoenician langugage, whose influence can be found in the richness of this prophet's speech..."Beshelmi" (Yonah 1:7), which came from the mouth of one of the sailors. This is a clear Phoenician term..." (pg. 205-207)
"Like all of the prophets, Ovadiah prophecied the destruction of the kingdoms whow ere considered, techincally, vassals of Judea: Edom, Peleshet, Efraim, Shomron and Gilad..."S'farad" is a word with a Phoenician structure, with a Dalet at the end (like "Rafsad")..."Ketel" is a northern Semitic and Aramaic word...by Yonah, whose birthplace neighbored the northern border of the area of the Hebrew language...we find more than any other prophet, words taken from Phoenician and even Assyrian...like "Meshamrim" instead of "Shomrim", "Lo" instead of "Oto", "Charishit" instead of "Me'alefet"...by Hoshea and Amos the differences in the north are more defined. Especially we find by them Shin instead of Samech...for example, בשורי instead of בסורי; and השירו instead of הסירו. And by Amos we find בושס instead of בוסס, and בולס instead of בולש, and מסרף instead of מצרף. And in Hoshea, און instead of הון..."Anach" (Amos 7:7) is a strange Sumerian word which entered the Hebrew through Phoenician, and the same about Haharmonah (4:3). From the common language of the north we find "Doga", "Avtit", "Kaluv", "K'vrah"...
By Nachum we find Assyrian titles for officials - "Tafsar" (3:17)...likewise for "Hutzav" and "Gulta"...
At the time, the richness of the language in Judea reaches its peak. The first in the line of southern prophets is Yoel...by Yoel the word "Acar" similar to the Arabic verb "Acar" - to farm the earth. By Yoel we find for the first time the word "Kerem" and whole slew of farming terms: "Mamgerah", "Magrefot", "Dashah", "Magal"...the rest of the new words point to a relationship with the southern Semitic dialects spoken by the neighboring nations..." (pg. 300-303)
Therefore, it seems very likely that the Hebrew dialect of Judea became known over time by other nations as "Yehudit" - "Judean".