As we know it is a big custom to learn a daf of Talmud Bavli a day (Dafyomi).
My question is why did they choose to pick Talmud Bavli. There are so many other Subjects that could actually affect your day to day life.
In the olden days - as stated in Pirkei Avoth בן חמש למקרא - one would only learn Shas (Talmud Bavli) after becoming proficient in Chumash and Mishna.
One would then concentrate on Shas as it's the basis for Halacho.
Besides, it's really difficult to properly understand Rambam, Tur and Shulchan Aruch (the 3 "off the page" pillars of Halacha) without having learned the relevant pieces on Shas. All the more so if one tries the Rif, Ran and Rosh who wrote "on the page".
So somebody who wants to live a meaningful - and Halachic - Torah life needs to become proficient in the DNA of Halacha - i.e. become proficient at Talmud Bavli.
Obviously, if one simply learns the Daily Daf and doesn't delve all the way into the practical applications of what one has learned, then one is simply "playing along" while missing the whole point of learning. (True for any learning program.)
To summarize: The point of the Daf Yomi program was two-fold:
Ensure all Jews are learning the same Daf - thus making it easier from travelers to continue learning wherever they land.
Ensure everybody can learn the entire Shas in a sensible time frame; thereby ensuring they cover all practical aspects of Jewish life every 7 years.
It was self understood that after/while learning the Daf one would continue on to its practical application.
In the introduction to the Shulchan Aruch, the Mechaber, HaRav HaGaon Yosef Caro Z"tzl explained that his work was meant to be learned over a period of 30 days. (lol)
Thus, from a stand point such as that, the basic source of psak halacha shouldn't take more than a month or so to actually take in at least on a basic level.
The Talmud on the other hand can be learned with extreme depth (i.e. machlokes Rishonim, points by Acharonim, psak halacha) and forms the cornerstone of not only our legal code and explanations on how to perform the commandments, but also is full of midrashic materials that strongly contribute to our hashkafic (and even mystical) understandings of the realities of existence. Hence, the Talmud is something that is far beyond simple psak halacha (something that as mentioned above was indeed meant to be completed quickly).
This is an answer based upon a pshat type of fold, the great mystical significance of learning the Talmud as brought down in the writings of Rabbi Nachman from Breslov, and other Mekuballim is also something to consider.
[Rabbi Meir Shapiro, the originator of the idea] saw daf yomi as a tremendously necessary unifying factor among the Jewish people. He said that if you look at a page of the Talmud, you see the words of the greatest Jewish minds covering the entire spectrum of Jewish life and history: There were Jews from the Land of Israel and Babylon; from France (e.g. Rashi) and Germany (the Baalei Tosafos); and in the back of the Talmud there was Rabbeinu Asher, “the Rosh,” from Spain, the Maharsham from Poland and the Maharam from Lithuania. He said that a page of the Talmud is the most universal and unifying fact in Jewish history because everyone great from every place in the world in history are all gathered in a single place.