[quote name='SDC']I think for most people Mario or Sonic games are platformers, while the modern sidescrolling Castlevania games are the wonderful category "Metroidvania" (or, rarely, "Castleroid"). The exploration of a giant non-linear map makes it far more than what most people consider a platformer.[/QUOTE]
Because instead of multiple levels, the entire thing is one continuous level. Which, in and of itself, is a downright GENIUS design idea, as it allows for backtracking, which theoretically makes them able to make a LONGER game with less in the way of levels.
Nobody I have ever seen has claimed that Metroid and the Metroidvanias weren't platformers.