One problem with the Hosuton - Longview bus (in my understanding, and I may be wrong) is that you can only use it in conjunction with a rail trip.So that basically means the bus is missing out on the short-haul traffic that always helps fill a handful of extra seats on a train.
And it is not only that you need to take a train to catch the bus, but you need to do so via Longview. So for example if you want to go from New Orleans to Galveston, the most logical thing is to take the Sunset to Houston and then the bus into Galveston. But no, Amtrak insists on routing me via Longview (and going via Chicago to get there, that is almost three days of travel for something that could be done in about 8 hours). If you really want to keep people off a bus, this is the way to do it.