Because there are not enough HOVs and the access to HOVs suck for the trips that most drivers want to take. As it is now, we barely exceed the capacity of one lane as is shown when you see a 3+ restriction like on 290 HOV in the peak hour.
If you could get more people to car pool it would could work, but carpooling only really makes sense in the peak hour when people are going to and from work.
TxDOT will not build any more single lane barrier separated HOVs. They will not extend the system. They may rebuild as is. The more preffered option is a diamong lane in each direction like you see on the suburban freeways. These do not really work in the urban area where METRO wants access via T ramps to their park and rides.