Threads never released in the EU in the first place, so this absolutely is not the reason for lowered engagement.
In an indirect way it could be though - not having the entirety of the EU on Threads is a huge non-starter for many people, as many of their favourite influencers, celebrities, companies, etc will be from the EU who were never able to get on it.
It's pretty hard to start working in the EU if you look at the absolute pile of data that Threads demanded. The GDPR paperwork to make sure they can get away with that AND still comply to GDPR would be more than the whole internet.