The few M&Gs I know of resulted from a line pilot's recommendation. Also, I can cite at least one case where a line pilot's recommendation was all it took to get an interview.

I don't think anyone knows why some get called, and some don't. Of the poolies I know, there are some with extraordinary qualifications. There are also some great guys and gals out there who would, no doubt, be an asset to UA. I hope UA continues seeking those applicants. Keep in mind, some of the poolies have been trying to get on for 10+ years.