Darth Maul=Massive Phail. Darth Vader=Epic Win. It's as simple as that.
Darth Maul is nowhere near as iconic. Plus, Darth Maul didn't grow into one bad-ass dude from an angsty little teenager. Nor did he rule the biggest empire in the history of man, wipe out an entire race of elite soldiers or father the greatest (and last) Jedi ever.