I think much of it falls down to ideology and who represents your ideals better. Even if other than words, there is no real effort to follow those ideals and the candidate actions go against them.
There is also a part which is the fault of the system itself. The western political system flaw is that in order to get elected you must make deals you don’t want to do, shake hands with people you abhor, and make promises you have no way to keep, just to get elected and make a change, that there is no way you could now make.
I don’t see the system change anytime soon, though, so most people are left with voting for people they don’t really believe in or voting for the opposing side, just because they really, really don’t like the popular candidate.