“since when do yidden do things out of convenience? ”
Frequently. Many mitzvos are locked in to a specific time and place. There is NO explicit halacha that governs the how, when or where or a siyum for daf yomi (or any other “siyum” for that matter). If the gadolim (represented by Agudah) decide that for reasons of cost, availability, safety etc. etc. to hold the siyuum on a secular holiday, thats fine.