Because Cartman is a racist piece of shit. They literally said it in the episode

I would suggest doing research into the meaning of pandering and compare it to diversity so you can form your own thoughts on it, but I will write out the intentions that I saw.

Using the pander stone was a metaphor for being lazy and merely casting diverse people to play shallowly written characters, and replacing people for no good reason (hence Kathleen wanting to put a gay chicken in Bambi was brought up), just to look good, check boxes, appeal to certain groups but though they initially had good intentions, they forgot that representation is only good if it has real relatability, depth, and purpose. Don’t believe me? Just watch the scene between the real Kathleen and Cartman, she literally apologises for using the pander stone (a metaphor for pandering) instead of putting the work in to incorporate diversity in creating movies that people will enjoy, true passion and hard work to an art and characters that people will really love. And then Cartman (the racist) also apologises for his laziness of not giving real critiques and ideas but instead writing racist and hurtful hate mail. Hence making it a touching moment (and rare from Cartman) since both sides can now see the others point of view and are more receptive to it.

There’s also the scene where I think it’s Pc Principal (could be someone else) is lecturing the boys on not accepting the new Cartman replacement, and then lists movies of diverse casting claiming the boys must be against them too, but Stan disagrees and differentiates that those stories have their own backbone and are their own established stories / franchises as opposed to casting say a black person as a traditionally white established character for the purpose of pandering.

So in conclusion, Matt and Trey are against lazy inclusion bait films, not against diverse casting. They want the joy of what movies used to be to come back, where people aren’t trying to tick boxes but instead write compelling and well fleshed out characters. They couldn’t care less about what colour their skin is or what their sexuality is.