jetzBy having 5/6 players be directly combat and aim-oriented you're going to attract the kind of players who aren't necessarily interested in the gameplay medic has to offer
yeah, overwatch has a much bigger need for support/tank players than tf2, but offers enough variety to attract people who like that stuff from the get-go. the gameplay is also, uhh, forgiving enough to allow people to have fun on support classes even in casual.
also, support players in overwatch have the option to easily queue for combat roles if they feel like it. even people who enjoy playing medic will tend to avoid pugs when they know theres a 99% chance they end up playing med every time