IMO, they didn't do such a great job developing some of the characters on Voyager (i.e. Chakotay, Kim, and Tuvok), and they pushed others in your face way too much (i.e. Janeway, Seven, and Torres). The Doctor was just the right amount that they didn't capture very well in the other characters. Plus Picardo is a great actor. I believe this was the case for Kes as well before she left.