You need to get your facts straight. Zuffa[parent company of the UFC] was not actively looking to buy out other MMA promotions. The promotions that the UFC ultimately purchase, were going to close doors if the UFC hadn't purchased them. And in all cases[WFA, WEC, Pride FC, and Strikeforce], they were the LAST people approached.
WFA - Was set to go out of business after their first PPV WFA: Street Kings because they couldn't keep the doors open. They actively sought out to sell the promotion, contracts, licenses. Zuffa was the last approached. That is how Rampage, Machida, Kampmann, and Matt Lindland all ended up in the UFC. Because their contracts were purchased by Zuffa.
WEC - Scott Adams and Reed Harris actively sought out other investors to invest in the WEC before they sold a majority stake in the WEC to Zuffa in 2006. Zuffa then kept the promotion open for 4 more years after their purchase. Zuffa was the LAST chance for Scott Adams and Reed Harris before they had to close doors because they couldn't afford to keep WEC open.
Pride FC - Rocked by the Yakuza scandal and being kicked off TV because of it, DREAMSTAGE looked to sell off its assets. Pride FC was for sale, and several investors looked into. WWE was actually in negotiations to buy it headed up by Shane McMahon who is a huge MMA fan/buff. At the last moment WWE pulled out because Vince McMahon felt that Pride FC would not be a good investment because he felt that MMA was a passing fad and would not survive for much longer. DREAMSTAGE actually approached Zuffa, and a deal was made. Zuffa had every intention of keeping Pride FC open as its presence in Japan/South Pacific but had pressure put on them by the Yukuza to operate as they did under the previous ownership. Zuffa shuttered Pride FC rather than be in bed with the Yakuza.
Strikeforce - Scott Coker saw an opening to get into the MMA game in 2006. After EliteXC folded in 2008, Coker actively sought out investors to make a move for EliteXC's spot with CBS/Showtime. San Jose Sports & Entertainment Enterprises stepped up. Coker sold a majority stake of Strikeforce to SJS&E to directly compete with The UFC. They signed Fedor to an unheard of deal, with the promises of PPV dollars in the future, along with several fighters to multi-million dollar deals. The PPVs never materialized, and while Strikeforce was turning a profit, SJS&E was not seeing the return on their investment that they thought they would be seeing so they actively looked to sell its majority stake of Strikeforce and exit the MMA scene. Scott Coker offered to buy back the stake, but SJS&E did not wish to sell it back to Coker. So Scott Coker couldn't afford to purchase it back. SJS&E even approached CBS/Showtime about purchasing its stake in Strikeforce, but they were not interested considering they'd been burned by EliteXC. SJS&E approached Zuffa even though Coker objected, but SJS&E wanted to get out with as much of its initial investment as possible.