A lot of people associated with Free and Open Source Software (FOSS) have major objections to GitHub. Here’s one summary: sfconservancy.org/GiveUpGitHub/
But the TLDR; version is roughly:
Your source hosted on GitHub is being used to train AI, and you are possibly giving up rights to algorithms you may have written (IANAL, and AI training is a fuzzy topic at the moment)
GitHub itself is proprietary, closed-source software, while they claim to be pro-FOSS. Aside from not being in the spirit of things, closed-source means you also don’t know what happens with your code/data once up upload it.
Microsoft has a history of being anti-FOSS, while some people will say it’s been changing, I think many are still rightfully concerned what their future decisions regarding GitHub might be, especially if they are a near-monopoly.
Alternative do exist, and some like codeberg.org are specifically open sourced, and pro-open source, so many people are pushing to move hosting away from GitHub and onto other options.