Because there is no guarantee that: A) Twitter will exist within a year B) Musk won't takeover or delete your account C) Twitter has been experiencing plenty of random downtime due "production experiments"/cost saving.

I also don't think it is intended to be a replacement, looks more like for help and general outreach; I mistakenly described it as 'swapped out' which I think has created some confusion about using youtube as a bug tracker.