I get Christine Chapel was not a very well liked character, heck, Majel didn't even like her. But one thing the Chapel I watched for years never would have done was jump ship because things went south with her love life or plans.
Um - remember - Christine Chapel CAME ON the original Enterprise (in TOS) because they were going to the planet where her fiancee (Roger Corby) was lost (see the TOS episode "What Are Little Girls Made Of"
). After they found the android Corby (and she lost him again) - she decided to stay on the ship.
My point? The character was often motivated into career choices via emotional issues if you go by that episode.
And if you remember her behavior where Spock was concerned, maybe she stayed on-board because she always hoped she'd have a chance with him.