Because of two things that I personally can see:
1. You're trying to set rules and limitations to something that, as the name says, is meant to be random. Dungeons have a requirement just because they are made for a group at a bare minimum point in gear. Battlegrounds are inherently gear-neutral. You aren't going to be facing a boss that requires this or that, you will be facing random players or groups of players. The very fact they are random should be enough to say they are something casual, that you're throwing yourself at whatever comes; and
2. How would that affect queue times? If we go by your examples, of how often you "faceroll" vs. how often you lose or have a challenge, how many people would rather wait that long to get queued with a "balanced" group instead of just having quick, casual fun? And where to draw the line? Is 25 item levels difference enough? Won't a premade with everyone 25 item levels higher still outmatch random disorganized people in the weaker gear just the same?