I think it comes down to several factors, especially for larger indie developers in that:
- The potential revenue of more popular storefronts may far exceed what they might get from itch.io
- Maintaining a game across multiple storefronts can be a lot of work as far as keeping up to date with builds, customer support, etc. that might ultimately not be worth the extra effort depending on the sales gained from such a place
Naturally in a perfect world, granting everyone all of the options they could want across a variety of storefronts would be preferred, but indie developers have constrained resources as it is so need to focus on what they feel is best for them, which in a lot of cases would mean not having a version on itch.io. There's also of course the matter of DRM, where some developers may feel safer having their game on Steam where it can't easily be distributed or shared around compared to more DRM-free alternatives.