Some of it, IME, is that webcomics tend to have a central cast rather than Protagonist with supporting cast, so it's a lot easier--as it were--to have a more diverse cast that's prominent and front-and-center. But I think a lot of it also that webcomics, being direct-to-audience media projects that tend to have very small (if loyal) fanbases, have (or perceive themselves to have) an awful lot more freedom to write what they want, and not only what they think will sell.
There's also a tendency for webcomic writers/artists to be younger overall than published authors. Hm. Now I am pondering all the reasons why webcomics would skew more towards diversity. Interesting.
I read only one of the webcomics you list there, but I could list lots more with interesting diversity within the cast. Questionable Content has queer and trans and disabled and PoC characters, Gunnerkrigg Court cares a great deal about the relationship between two girls, The Amazing Adventures of TJ and Amal is about two gay men (one white)... Heck, even if you jump over to Hark, A Vagrant, it's 90% history jokes, and actually notices that an awful lot of people in history are not actually 7Ps.