There isn't a solid reason.

It wouldn't affect premades that much, no one queues for current EX trials in DF so that wouldn't be affected and you gain little benefit over an 8 man premade besides slightly less competition on mounts, but lootmaster groups exist anyway.

It's probably just "to be safe" incase there was some reason they can't think of or some current instance becomes soloable due to an exploit they missed, and they see no reason to change it.