I think it's due to them being previously expired trial mechs, but I'm not sure. Haven't played the game in ages but I've been trying to get back into the swing of things now that there's some active devs.
I can play my Technician, Raider, and Recruit, but I can't seem to play the Infiltrator, Berserker, Grenedier, Bruiser, Reaper or Brawler. They all say they're expired trials. Which is weird, considering three of those are apparently current trials.