Kit difference.
Take Mei for example, probably the weakest hero in the game at the moment.
Apart from her ultimate, her overall kit is extremely weak, relying heavily on close quarter. Unlike most close quarter heroes tho, she has poor mobility and can't quickly get out of a situation.
Her Right click also has dropoff damage and is a projectile, making her all-in-all, weak and rarely played.
Compared to say Tracer, who has the power to quickly burst done any hero and quickly escape.