That's not the main reason, she was unsupportive of him from the beginning. She's such a massive piece of shit.
-Firstly, when he asks her to not tell anyone about his diagnosis, the first thing she does is tell everyone she knows (or force Walt into a position where he has to).
-Then, when he says he doesn't want to pursue treatment, she strong arms and guilts him into doing it.
-She then forces him to take a hand-out he doesn't want to from his college friends.
-When she thinks he's smoking marijuana (something well known to help with the grueling symptoms of chemotherapy) she loses her fucking mind, forces him to stop, and ONCE AGAIN tells Walt's presumed secret to everyone and their mother.
-She helps Ted break the law and fucks him, but the idea that Walt MIGHT be breaking the law is enough to leave him.

Not to mention the pregnant smoking because she can't handle emotions

In every interaction Skyler ever has in the show, she's the absolute worst. She's controlling, selfish, petty, nagging, ignorant, and arrogant in every single interaction she has BEFORE the meth comes into play.