Because the UFC isn't recruiting good talent. They're recruiting random guys to fill in the 10000 open slots on the 100000 cards. That makes them focus on getting bodies instead of talent.
For example, it's a common misconception that HW is dead. It's not. It's better than ever. The UFC HW division is dead because they're not focused on recruiting the best HWs in the world. The UFC HW top 10 is not the world HW top 10.
Do you really think that there aren't 20+ guys that can beat AA and Mark Hunt?
Most of a UFC card isn't watching the best fighters or the best up and comers. It's watching random low level WMMA, 125ers, and random guys -- not up and comers or exciting prospects. If I wanted to watch random guys, I can watch local MMA and not have to suffer through the WMMA or 125ers; that's the level these guys the UFC is recruiting are on.
Do you really believe that there aren't 15 better LHWs than Pat Cummins, for example? Is he really the 11th best guy in the world or is he just a random guy they got to fill in for DC? I'd rather watch the millions of guys that are better than him in non-UFC orgs than watch a free UFC card filled with guys like him.
UFC used to focus on having the best fighters and so all their fighters were worth watching. It's just a brand name at this point and most of their roster, and so most of their cards, are regional level fighters, WMMA, or 125.