Frieza, like many villains in the series, was simply a force for the heroes to overcome. His character pretty much ends at "greedy powerful alien", we never get any insight into him as a character, what motivates him or what made him what he is. That works fine on Namik and his worth as a character ends with his death. He doesn't have the depth to carry multiple storylines as a villain, so no matter how serious he is treated in the story, bringing him back is a joke because it is repeating the same simple character who has no place anymore.
Vegeta and Gohan are different. Vegeta also started out as a simple force for the heroes to overcome, but quickly played a very different role in the next arc. He was able to develop as a character after that, we get to see what has made him into who he is and he has changed overtime. Both him and Gohan are in a new position in the story, with growing families their stories are most certainly not complete.
Frieza so far is still just that force, and with the writers/story showing no signs of wanting to change that dynamic, he is not bringing anything new to the story. And I can understand why he hasn't changed after multiple chances, where is there to go with the most popular villain when he is mostly just an obstacle for the heroes, and wasn't designed to last more than that one arc?
Vegeta and Gohan have far more going for them as characters at this point. I don't think this is really debatable, but if you have something to say other than "you're just biased" then I'll gladly listen.
