There are many reasons why many choose to use Photoshop over Illustrator, but it depends on the particular situation.
As an example, I myself would use Photoshop when designing for the web where I need to be pixel perfect. I can't really work at the pixel level with Illustrator, so Photoshop is perfect for web work in that sense.
Now, I might have an instance where I wasn't sure of the exact dimensions of the final work as it could potentially be used in offline material. In that case I would probably go for Illustrator, because I can scale up or down without losing quality.
There are other reasons, such as the fact that vector based programs are less effective at reproducing gradients.
Also I agree with Rsiel;
I think there are more people who know how to use Photoshop, than
there are people who are comfortable with Illustrator.
There are those who are frightened of having to use bezier curves to get a perfect line, and from the outset simple tasks like filling colour look difficult to accomplish.