@ainmosni National servers probably make the difference.
Any US-American will have more problems with discovery of content (No search on Mastodon but hashtags). The choice of instances may be intimidating for someone - firehose of any big instance's local feed is too much and a small tematical one will amplify your FOMO. They want to see content tailored to their preferences, but isn't a thematical instance too limiting? (they don't know, that almost every server allows generalist talk).
A language barreer eases the choice. Almost every German state has a server (like Berlin), there are only several Polish instances, and so on. Local timelines are easier to browse on regional servers.
US-Americans have too much social mobility to have this European experience.
And Europeans are generally more comfortable with "national" social networks. I am writing this from /kbin, appreciated as Reddit-alternative on the Fediverse. However, its developer modelled it actually on wykop.pl - another link aggregator, much more popular in Poland, than Reddit.