Simple. The three plan (Internet, TV, Phone) all worked out to be cheaper and better. I got faster and more importantly, more reliable internet. When I was with cablevision/Optimum Online I would get maybe 5Mbit speeds that would flake out during prime time hours since they were over subscribed. Now I get 20 Mbit consistently, even during peek usage hours. The TV was a better quality image, more channels and more innovative products (Multi room DVR rocks). Phone is nothing exciting, but since we also have cell phones with verizon, we get a small discount for linking all our bills together.
Overall, I got the impression that cablevision simply stopped innovating since they were the only game in town, and they did not care that much about their customers. They sure got a big surprise from Verizon, and they are calling us up every week it seems begging us to come back.