I think the official line is that she resigned because she wanted to do more "prime time" tv.
I didn't really see the point of her either, not to fault her presenting ability but she didn't strike me as having any interest in or knowledge of F1. I think she was only ever going to get bored in that job.
That's one of the problems of picking someome for the "eye candy" they risk having someome who clearly doesn't know what they're talking about. ITV were particularly bad for it, for their fortnight delayed coverage of GP2 they sent along a bimbo who although looked very nice clearly didn't know the first thing about racing and asked embarrassingly stupid questions to the drivers.
They even did it for the F1 for a bit, they had Beverly Turner, who although obviously clever in her own right didn't have any interest in the sport what so ever. I always got the impression that she took the job because "oh wow look Monaco glitzy glamour pampering" then had culture shock in the actual behind the scenes reality.. Then got upset because no one took her seriously and decided that it was all to discriminatory.
That's why the likes of Lee McKenzie are so much better, because they have genuine interest in the sport, they aren't there because they think they might get the chance to rub shoulders with A list celebs, they know their stuff.