Is it because people don't want to say the word, or is it because they want to demonstrate that they believe women should have the choice of bodily autonomy? Just using the word abortion doesn't take this into account. When you stop to think about people who are anti abortion, or anti choice they literally believe that women should be denied the choice of the fundamental right to basic healthcare. I think the term pro choice shows the nuance in the argument. The word abortion alone limits the nuance.

Also, as somebody else mentioned in the comments, it's a basic medical procedure. People don't say they're pro quadruple bypass, so why treat abortion, a basic medical treatment, differently from every other medical treatment?

I'll say both. If somebody asks me if I'm pro choice, I'll respond that I'm pro choice. If somebody phrases it as pro abortion, I say that I'm pro abortion.