Raptors became good for a couple of reasons; their players got better and their division got crappier. Lowry, as gunner21 mentioned, became a top 2 point guard in the East. DeRozan (currently injured) became one of the top SGs in the East. Their frontcourt is pretty good too with Valanciunas, Patterson and Johnson & Johnson.
Then you have the demise of the Atlantic division.
I think they're still behind Bulls, Cavs and Wizards though come playoff time. Raptors still lack a bit of star power and have a questionable bench. Bulls got depth, Cavs got a Big 3, Wizards have Wall, Beal and a better PF/C rotation.