A lot of people associated with Free and Open Source Software (FOSS) have major objections to GitHub. Here's one summary: https://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.

The previous Microsoft's CEO truly hates FOSS, famously calling it cancer. Then the next CEO reversed Microsoft's stance on FOSS, acquiring the largest FOSS collaboration site. Naturally, many view this move with suspicion since Microsoft has a history of embracing something only to extinguish it later.