Dear Ron,

In our "Introduction to Maseches Shekalim" (which is available in our Archives at www.dafyomi.co.il, or on request we will send it to you), we addressed your question why Shekalim was included in the Dafyomi cycle. The most simple reason given is that since Shekalim is the only Masechta which does not have any Gemara Bavli on it, Rav Meir Shapiro included it in the cycle in order to make Seder Mo'ed complete. For the same reason, he included Kinim, Tamid, and Midos in Seder Kodshim, even though there is no Gemara on them.

As to why he did not include the two Masechtos in Seder Nezikim which do not have Gemara (Eduyos and Avos), it is not clear. It could be that he did not include Avos because of the widespread custom to recite the Mishnayos of Avos each Shabbos between Pesach and Shavuos (and some continue until Sukkos). Perhaps he did not include Eduyos because most of that Masechta contains Mishnayos which appear in other Masechtos.

All the best,

Yisrael Shaw