I think that Stan takes a lot of heat because he was a shameless self-promotor. He didn't put as much effort into promoting his collaborators, but he wasn't obliged to. I'm not aware that he ever actively denied them credit.
Also, while Kirby, Ditko, and the other artists certainly deserve their share of credit for the creative process, there was one common element to the creation of the classic Marvel characters: Stan Lee. Likewise, his writing and his talent for promotion set the entire style of the Marvel line as it was growing to overshadow the competition. Kirby and Ditko weren't writing Bullpen Bulletins or the letters pages.
Also, while Kirby, Ditko, and the other artists certainly deserve their share of credit for the creative process, there was one common element to the creation of the classic Marvel characters: Stan Lee. Likewise, his writing and his talent for promotion set the entire style of the Marvel line as it was growing to overshadow the competition. Kirby and Ditko weren't writing Bullpen Bulletins or the letters pages.