The queue times for the PBE are pretty horrible. I was just waiting in normal solo queue for 40 minutes. Eventually, I joined public chat and found a few people to play with. It's a little hard to test when there's only what seems like 20 people playing on it at a time.
Maybe Riot could offer some sort of incentive for people's normal accounts if they help test the PBE. Aspecial skin, an IP boost, RP, IP, something not too huge but enough to get people to play. Or maybe you guys could advertise it a bit more.