Because trains don't get filled from Mysuru & Hassan, my parents used to live in Mysuru I regularly travelled from Pune to Mysuru by all routes possible, Trains via Hassan don't get full until Belagavi, I recently travelled in Mysuru Nizamuddin Swarna Jayanti Express and experienced this myself.
Most of the rush is for Bengaluru only, that's why many trains run via Bengaluru.
Fun Fact : Many northerners don't even know where Mysuru is, they just take the train for Bengaluru.
So...
more... it's not only about bus lobby, it's about demand too, there's not much demand from Mysuru, Hassan towards North in long distance trains & vice versa. Major demand comes from Bengaluru only.