It seems to happen time and time and time again - soap stars who are actually pretty decent actors rise to the top of their field, leave, and nothing becomes of them.
I think it's 'big fish in a little pond' syndrome - they get a lot of popularity, dominate the various soap awards, and some of them suddenly think Hollywood must be the next step.
Problem is, once they leave, they're already ingrained in the publics conscious as that character that they struggle to step out of the shadow, or realise that despite being superb by soap standards, when it comes to film and more 'acclaimed' tv drama, they are still not quite up to scratch.
Thankfully, there are occasionally those who break the mould. I think Sarah Lancashire is a fantastic example of a popular actress who left her soap at just the right time and has plenty of fantastic, reputable projects and picked up a slew of major plaudits and awards along the way (including, most recently, a BAFTA).