Because it's a level 49 trial, the pool of players that both need and will actually queue for it is pretty low, and with it being low level, there's very little incentive for people who've done it already to queue for it. By making it main scenario you would at least get people trying for their daily philosophy reward into that pool so players who need it can actually get into it in a reasonable time(would probably require raising it's level to 50, but I'm sure all the people waiting to do it wouldn't mind).