There's a few reasons why more companies don't sell this technology.
The main advantage to using MoCA is the simplicity of the setup and ability to reach remote devices (outside of your Ethernet/wifi range). However, MoCA is mostly proprietary to cable provider equipment, whereas HomePlug (powerline networking) is readily available through a number of vendors and retail devices. Finally, in terms of speed, networking through MoCA is much slower than either gigabit Ethernet or a good Wi-Fi connection using wireless N or AC.
The proprietary nature of MoCA means that if you switch providers, you need to switch networking equipment. Slower speeds (compared to CAT5e or WiFi) means that you're unlikely to use MoCA unless necessary. And HomePlug probably does a better job at connecting distant devices, since a room is more likely to be wired with power outlets than cable. And lack of retail (non-ISP) products limits the usefulness of the technology.
Note that the same arguments apply to HomePNA phoneline networking offered through DSL providers.