Bs"d
Why & when was a Yerushalmi masechet put into the Bavli daf yomi cycle?
Thank you,
Avi, Jslm, Israel
Here is what the Kollel wrote in our introduction to that Masechet regarding your question
Be well,
-Mordecai
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INTRODUCTION TO MASECHES SHEKALIM
brought to you by Kollel Iyun Hadaf of Yerushalayim
Rosh Kollel: Rabbi Mordecai Kornfeld
daf@dafyomi.co.il
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[I] GEMARA (YERUSHALMI)
(b) Maseches Shekalim is unique in that it is the only Maseches of the Yerushalmi that is printed in the common editions of the Shas Bavli, and is included in the Dafyomi cycle. The obvious question is, why? Although the Mishnayos of those Masechtos for which Gemara was not written were traditionally included in the Shas, it is puzzling why the entire Gemara Yerushalmi was included in this instance, as opposed to the Yerushalmi on Zera'im. Perhaps it was included since it is the only Maseches in Moed for which a Gemara Bavli was not written, whereas no Bavli was composed for nearly the entire Zera'im. Since it meant only a few extra pages, the otherwise frugal printers decided in the case of Shekalim to include the full Yerushalmi and not just the Mishnayos. (For unknown reasons, Yerushalmi Horayos is also included in some printings of the Shas Bavli, perhaps due to the brevity of the Bavli on that Maseches.)
A harder question to answer is why the founder of the Dafyomi cycle, Rav Meir Shapiro, zt'l, included it in the cycle. His intention may have been to complete Seder Moed -- a hypothesis confirmed by the fact that he included Mishnayos Kinim and Midos in the cycle to complete Seder Kodshim -- but then why did he not include Mishnayos Eduyos to complete Seder Nezikim? (Avos was understandably left out because it is learned in either case, between Pesach and Sukkos.) Perhaps this Maseches, and Kinim and Midos as well, were included in order to fulfill the teaching of Chazal that "When Talmidei Chachamim study the laws of the Temple service, it is considered as though the Beis ha'Mikdash was rebuilt in their days" (Menachos 110a).