Previously it was free and the noises that GitHub had given made it sound like it wasn't finished. Turns out it was.
But it gets worse: because you can't tell whether copilot is giving you illegal-to-use code, even just looking at its output can make you liable for future transgressions because seeing copilot code may expose you to license-encumbered code that you might now use as inspiration for new, non-copilot code. Even if you in good faith believe your new code is your own, the law does not agree with that assessment: if your code is similar to something license encumbered that you read in the past, even if you forget about it, you are now potentially in legal trouble.
Simply by using copilot at all, you're taking on a risk that is great enough to go "actually, I am not even going to try this thing".
You don't know me, but it's totally fine to drink this this mysterious blue liquid. I pinky promise it isn't toilet cleaner.
Another critical reason is that GitHub is proprietary and has a pile of services with lock-in that (unlike git) aren't easy to move elsewhere. Some people depend on GitHub for their livelihood (through GitHub Sponsors). Some people's software integrates tightly with GitHub bots, or Actions, or issues, or project management. Everything other than the code itself is incredibly difficult to port over to another service.