A lot of sports fans don't want to listen to every Phils games, so I think it makes good sense for CBS not to break format on WIP when they have another signal available. That said, as a talk fan and not a big sports fan, preempting shows on 1210 always sends me elsewhere. But since 1210 is the only major talker in Philly, CBS has the luxury of getting their talk audience back after the game - where else is a terrestrial radio-only listener going to go for mainstream talk if they don't prefere what's on WNTP?
WPEN could easily put the Phillies on the AM while sticking with ESPN or whatever else they're doing on the FM. The PEN(AM) signal isn't nearly as good as 1210 of course, but it's serviceable as the flagship Phillies network outlet. If there are holes where 1210 went that 950 doesn't, suburban affiliates can fill them.