This will not be easy to explain in English but I will do my best.
First, let's be honest (and a bit blunt!) about something: a cock is a cock. You either appreciate them or you don't. Ok, some are nicer than others but if you like giving blow jobs, you can still enjoy (to a certain extent, of course) doing it with someone who is not really attractive since you concentrate on a very specific area of the body.
Kissing, it's a whole other story. What do you kiss? A mouth... which is part of someone's face. If that someone turns you off, it's very difficult to make abstraction of it because the second you open your eyes, what do you see? His face. Which will remind you automatically that you're performing a very intimate act that you would rather not be doing.
Also, kissing involves an exchange of fluids. It can be gross to do that with someone you find unattractive. Giving a blow job does not necessarily mean an exchange of fluids. Yes, there is a bit of precum but it's usually very discreet, so much so that you barely notice it.
Now, just to be clear, I personaly love kissing, to the point where I'm disappointed if my date is not into it. Most of the time, I'm lucky enough to pick the right partners for me and even when I don't, I can usually find at least a couple of things I find charming about the person I'm with, which is enough for me to feel comfortable.