It's so hard. Someone can be the best actor in the universe and it's still and always will be a case of getting them lucky breaks, being in the right place at the right time, knowing the right people and in days of yore, the casting couch 
When a plum job comes along, it's the bee's knee's and once you're used to earning a certain income and living a certain lifestyle, is hard to give up. Unless you're willing to take the jump off the cliff. But it's rarely successful. Think Martine McCutcheon. Riding high in soapland, decent role in Love Actually (not saying she did a good job mind...), now advertising yoghurt
The list is endless.
Also it's not as if you have to be a great actor to do soaps (or any television/film for that matter) what with the amount of re-takes at your disposal. Give me the theatre any day. Real, live, honest to goodness real time acting. No room for error, no restarting again, raw emotion, a live audience. If you fuck up then it's up to you and the rest of the cast to use your experience and get yourself out. No saying "Sorry, I forgot my line can we do that bit again?"
Any successful television/film actor who puts their money where their mouth is and takes to the stage gets my vote
Even better if they do a Shakespeare.