When it was started it also was not wheelchair accessible. Not sure if this is still the case. Cherry picking profitable routes is not the same as providing access to all residents of the city.
Common carriers are a long-standing practice that has clear benefits for society as a whole. What's more, back in the 1960s and 1970s we -as a nation- realized that there is value in helping the weak, infirm, disadvantaged, and those subject to discrimination participate as fully as they are able to in the public sphere.
If you want a somewhat less academic reason, remember that you can be afflicted by illness, infirmity, handicap, or -albeit on a longer timescale- discrimination at any time. When you're down, it's good to have services available that help prevent you from hitting rock bottom and allow you to help yourself get back on your feet quicker than you would on your own.