Because most of the DirecTV type internet connections use a modem for the uplink, and the satellite dish for the downlink, and provide a very asymmetrical connection. There are some satellite dishes that can handle both up and downlinks...but I don't believe that TracVision makes a dish that can support it yet.
While that used to be the case (needing a phone line) I dont believe the current generation of Hughes home internet plans require a phone line for their internet access. I think the limitations is frankly related to the constant movement one encounters on a boat. Satellite communication needs stabilized equipment for clean uplink/download capability.