He had or retained the title 4 times, he had 3 title fight wins, and did 2 in convincing fashion in the deepest division in MMA. Plus the last fight against Henderson (which was ruled a loss for Edgar) was a toss up.
Add in that the top FW contenders (specifically the Korean Zombie) were injured, and this fight makes sense.
Plus, it's not like Randy Couture didn't get an immediate title shot at a different weight class. By winning all those title fights, both Couture and Edgar earned instant title fights.
Maynard/Edgar 3 was Edgar bell to bell in a "convincing" fashion, wasn't it?