Cas left Dean behind for Dean's own safety. He knew that the monsters would flock to him, and if Dean was as far from Cas as possible, he would have less trouble to contend with. Sam was never in Purgatory, and he had no idea that Dean was in Purgatory.

Not only does Dean die on a routine vampire hunt, passing away slowly in his brother's arms (which coincidentally may be the best scene in the entire final season), but he's briskly whisked away into Heaven solo.