Because you'll be the only one in the cab? Plus you don't have to wait long for cab. I don't like to take cab because it's too expensive. I'm thinking of buying a car next year so it'll be easy for me to move around
Part of it has to do with the fact that buses are usually part of public transit which is subsidized by the government. Also obviously travelling the same distance with only one person paying versus like 20+ people paying.
Because in a bus, many people fit in one vehicle so you all can pay a small price. In a cab, you're one of the only, if not the only passenger paying the price for the ride.