The scenario population is currently imbalanced

[DiMakss]

Every 2 sc for last 2-3 weeks.
As sc oriented player, i should say, something is broken, never was so bad before.

[agemennon675]

Probably people who got the scenario pop didnt accept it and there was no one to backfill the scenario, this happens a lot

[skylinx]

been like that since last patch.

they did advise that the scenario manager would try and create more balanced groups. the devs were worried that pops would be slow, especially for dps. on my destro toon you could wait 30 mins for a sc...

however,

NOW it looks like they have relaxed the criteria for sc pops to make pops easier and quicker. im now getting pops within 5-10 mins but massive composition balance issues and population imbalance, probably just needs tweaking a little

although theyve yet to admit any issues...

[Toonman]

Been playing since T3 was the cap and it's never been like this in SC before. Every other SC ends early due to this. At least mid-tier. It's awful.

[zulnam]

I felt this as well. Much more often than before. It most likely has to do with the recent tweaks to SCs in order to be more balanced.

My belief is that even though there are spots left, the new logic restricts other players from joining since it is anticipating filling the spots with missing roles (usually healers or tanks). This is just anecdotal, I have no proof that this is the case. But like OP said, I too have noticed SCs ending due to pop imbalance much more often.