Because trying to test any actual gameplay on the US server for those of us in the EU is prohibitive at best, the difference between a 20-50 ping (EU server) and 250+ (US server) means that little testing can be done on weapons or actual combat meaning that the test server for most of us non stateside is only any good for looking around, and not in fact taking part in any decent play testing.
Usually there is only a handful of people on it. I doubt maintaining an EU based test server would be worth the time and money.
Becouse : Ping, ping, ping, ping and PING! This is not good to fighting with US players, becouse i have 200+ ping and they crying becouse i'm WARPING or TELEPORTING
Sadly, it's a case of this. Outside of focused playtest, server smash use, or people just previewing mechanics/base changes. It's not heavily used to begin with. I'm not sure I'd be able to justify two of them if I were them.