Mauer also never spent his entire career as a catcher. He moved to 1B and DH after, so that could go against him. As well, he never won a World Series and had durability issues.
Guys like McCann, Posey, and Martin played behind the plate until they retired. Same with Posada.
There is case to be made that Mauer could be in the HOF. He put together some great seasons and was one of the best pure hitters in baseball when healthy.
John Olerud I think deserves a place in the HOF if Mauer does but he only played 1B/DH. Though he did win two World Series and had two 8.1 WAR seasons.