A couple reasons I would imagine. Adding to the building would have cost more money and time and they are looking to get this open before the summer season. Doesn't make sense to open a new ride going into a slow season, you want it in a busy season to pull people from Disney and come see Uni for a day or more.
Other reason is that these are extended queues and some day will no longer be needed. Yes, the one under the awning will, but that one even looks more permanent with the TV monitors, but I am thinking the one by monsters will not be needed forever and this would then allow them to remove it and give it back to monsters cafe or whatever place to eat is there at the time. Yes, in peak seasons it may always be needed, but they could add and subtract it for the season vs. having something that will always be there.