Honestly, because I think they realize they rushed the decision and look pretty foolish. It's better to make a U-turn now, than go another 300 miles on the wrong route. While a school like Ohio State can likely withstand the negative fall-out, I'm not sure most Big 10 football programs can.....I know academics are Big 10's top priority, but crushing their football programs, isn't sustainable long-term for the rest of the athletic department budgets.
They look pretty foolish when HS's are safely playing, and colleges all over the country, with far less budgets, are also safely playing. The players and families are begging to play, and have relinquished on a host of demands previously made. And lastly, Tom Mars says he will go away if Big 10 plays in Fall. Otherwise his goal is to make life hell on them with FOIA requests (I'm paraphrasing him, but he's fighting like hell for the players).