The people leaving Twitter want:

1. Both high quality and high quantity posts instantly available

2. Reasonable API access, maybe not directly but they want their niche workflows to be supported

3. Clear and consistent moderation. Eliminate disruptive content while giving reasonable people a clear understanding of whether something will be removed before they post it.

Threads has made vague promises to these three but not yet delivered.

Infosec and similar communities migrated off twitter starting months ago.

Threads was not an option then, and isn't of much interest to these communities (as a platform to socialise upon) now.

Threads lasted for like a week

Not available officially in Europe, for one

not available in EU.

Threads is too new for large communities to coalesce there yet.