Maybe that's because $2.50/mo/show is too much. Also, paying by the show doesn't give you the ability to check out a show on a recommendation that you may or may not want to really watch.
That's a lot cheaper than cable for most people. People pay > $100/month for cable, which would pay for 30 - 40 shows from Season Pass. How many people actively watch more than 30-40 shows?