NFL teams like the Patriots (on WBCN 104.1) wanted to take advantage of FM because of the stereo sound quality (crowd and player-contact noise) and also they linked with a station that had a strong following among young adults (The Rock of Boston = N.E. Patriots Rock Radio Network). The football plugs the rock and vice versa.
FM baseball? You'd have the crack of the bat and crowd noise in stereo, announcer's voices in stereo, and "no static, no static at all" as Steely Dan once put it. During thundery weather it can be a challenge on AM (esp. a weaker/faraway FM signal) to hear the game with all the static. Of course FM does have limited range in terms of line-of-sight. This is why, for example, WFNX in Lynn/Boston, MA (101.7) had to put up a repeater at 101.3 in Boston itself (reg. transmitter is slightly north of town) to aid reception (but now that WFNX is moving its signal to the top of the "Pru" in Boston, the repeater will no longer be needed).
Sometimes you can get a good solid signal on FM but yes, there's the line of sight problem. Ideally it would be great to have both and AM and and FM carrying games (most likely co-owned)...Entercom, which owns WEEI (850) could also run the games on its sister station WMKK (93.7). And starting this year, WEEI-FM (103.7) from Westerly, RI, which has a killer signal, will also carry the Sox.