I can't think of a lot of masked wrestlers who really had a lot of success in the first place. The ones that come to mind right away are Kane, Rey Mysterio, Abyss, Mankind & Vader. Are there really any more masked wrestlers that enjoyed a great career while wearing a mask that I'm forgetting?
Mankind shouldn't really count, although you could argue that Mick Foley won the title as that character. It wasn't the mask though, Mankind would have been a great character with or without the mask. Also -- If Mick Foley had only wrestled as Cactus Jack/Dude Love/Mick Foley (as himself), and never implemented the Mankind character/mask, he still would have had a stellar career to look back upon.
Kane worked quite well without his mask as well. One could even argue that Kane had a better run without the mask...although the jury's still out on that, since he just brought the mask back.
There are countless others who have worn masks, who didn't amount to jack squat. Guys like Juventud Guerrera, La Parka, Glacier, Mortis, Psychosis (he wore the mask in WCW), and there are many more that didn't achieve the small level of success that these guys had. There were some masked wrestlers that achieved a small amount of success with ECW and WCW's Cruiserweight Division, but not much more.
It's like others have already said, it's a combination of the wrestler's gimmick and the wrestler's ability (both in the ring & on the mic) that get a character over in wrestling...not to mention their size, or "look". It's not the wrestler's ring attire, or a piece of fabric/leather that they pull over their heads. They also need that "X-Factor". All in all, the sob stories definitely outweigh the success stories when it comes to masks...at least when it comes to Professional Wrestling in North America.