If we want to talk gimmicks instead of community curation:
Something I believe may be happening is a sort of "search engine optimization" (SEO) problem. The server browser allows folks to sort by players. If they sort by that, a higher count often appears higher if the server contains that many players. It doesn't sort by total slots, though. So with 40 being a bigger number, it appears higher in a way I imagine many users may be using the server browser. If that's the case, well, sorry -- that's a bad thing to optimize for because it ruins the game most of us are seeking to play as proof by the receipts Puuro provided about the Turbine+EU server (bigger number =/= longer life). Find a different way to defeat the evil server browser/valve being shit at supporting community servers than this gimmick.
I haven't done a search, but what happened to the betting plugin? That was a great feature that caused folks to focus on gameplay, want to stick around, and caused conversation topics that didn't suck. These are the types of gimmicks that should be added that make it worth playing on your servers. Fun cushions around the vanilla-ish TF2 that we all love. These grow people's interest in returning to a fun place to play and talk. I'm a dev and community builder myself, and would be happy to talk or get some designers together to figure things out that could be done if engineer or management power is something you need help with.
Also, please listen to the calls of my EU friends as well or I suspect the issue will cannibalize them away and be even less likely to fill the US server sooner when they move over.