Why were the Dempsey-Tunney fights scheduled for 10 rounds?

I think Chicago would have been out of the question in 1926.

The capacity of each of the two Chicago major league baseball parks at that time was small. Wrigley Field seated 14,000. The much larger Comiskey Park seated 28,000. Both were being expanded, but would never be giant stadiums. Soldier's Field had been initially constructed under another name in 1924 but was in a process of a vast expansion and re-dedication as a memorial to WWI soldiers, hence Soldier's Field. But the stadium was not officially opened until November 27, 1926, for the Army-Navy game with an attendance of at least 110,000.

I don't think there would have been any chance for the Dempsey-Tunney fight being the first event at Soldier's Field considering its dedication and Dempsey's rep as a WWI slacker.

So Philadelphia and its huge new stadium was the obvious pick.