Matchmaking System

From wow.incgamers.com

Blizzard has made matchmaking systems for their PvP games Battlegrounds and The Arena. When you enter a queue for either of these games, the system will look for other players of similar level and group composition, or Team Rating, and match you with as equal opponents as possible. The system starts rather discriminate, but quickly broadens the search pattern to include all available players.

Should you find yourself in a very long queue (more than 5 minutes), there just isn't enough available players to fight against/alongside.