Bale won both the PFA and FWA gongs ahead of the Dutchman.
Merson wrote in the Daily Star Sunday: "For me, Robin van Persie has been the season's outstanding player by a mile and how he hasn't won either of them is beyond me.
"He's scored more goals than Bale, has earned more points for his team and basically won Manchester United the championship - while Spurs are only fifth!
"I find it extraordinary. I mean, what more do you have to do?
"Don't get me wrong, Bale's a great player.
"He's scored some great goals this season and looks a class act every time he goes on the pitch.
"But Van Persie has won his side the title."