Money. It's cheaper to be in the transmission business than the content business. Same thing with telecom.
Actually, cable/satellite providers are increasingly more interested in being on the content side of the industry. Just look at all the regional sports networks that cable companies own. They own those sports rights, won't be at risk of a carriage drop of valuable sports content, won't have to pay an additional fee to carry that content, make money off that content from carriage fees from other providers and are able to control ad revenues from that content.
In fact, if you look back to before the NBComcast merger, Comcast had already operated a few of their own channels- many Comcast Sports Net channels, Versus (now NBCSN), E!, Style, G4 and Golf Channel.