UFC Didn't buy strikeforce and pride, Zuffa did.
But with MMA dying down these last couple years (champions getting old, new champions can't stay champs, fights are no longer worth buying) they need some where to put their "bad" talent and still make money off it. If you lose 2 or 3 fights in the UFC / WEC you are most likely being booted and going back to fighting at smaller / local shows. But when they buy these "smaller but bigger" fight companies out they can still have Hendo fight Jackson with out losing money and letting fans get less expensive fights.
Honestly, I quit MMA almost a year ago due to lack of interest / compeition. Everyone basically fights the same, nothing you see is new anymore. Everyone knows BJJ, MUAY THAI and Boxing so it's like a chess match rather then a fight. In fact if you want to see or be in a real fight, go to a gym (griffonrawl, strongstyle, ect..) and spar. You get better fights sparring then you do in the cage when it matters.
A lot of my old fight buddies I trained with for years have lost interest as well.. I think it's a fad like Pokemon or something. I havent even bought the last few fights on PPV, it's all the same now.