mary was under much pressure when she played fed cup than in any other games. i think that's why she doesn't want to play unless she feels she could give france a win.
Forget might understand that. they've played mix doubles together, and keep in touch for a long time. but many of her teammates, like loit, definitely want to win the title without mary. they don't know mary that well, they have never won a big title, and of course, they're not half-american. they might think they are the only ones who really care about fed cup, and wanna catch every single chance to convince that mary is just a stupid, selfish american.
i guess mary has noticed that attitude, and might be even more careful about her decision - they might blame her even harder if she played badly and lost
fed cup is definitely not a fun trip, especially when there's no friends left now.