It is available outside the US, in most regions other than the EU (which is due to concerns about violating the DMA afaik)
The GDPR requires data protection and it mostly goes against Metas business interests to do so.
It's not required to store or process date in Europe (im quite sure "serving" is not a problem as Mastodon instances can legally operated here).
And anyway Meta has plenty of European resources.
For now. If it's a success it'll come in due time.
"Meta is waiting for more guidance around the Digital Markets Act, new EU competition rules that govern how large online platforms use their market power, a person familiar with the matter said, "
DMA is about large companies and not the tiny mastodon instances.