Because:
1) they have to allow inbound traffic for those coming from far away to evac relatives who can't drive out
2) it takes time to clear out those southbound lanes and get all the entrances blocked by patrols, the contraflow will go for 100 miles
3) it won't start until the north side of Houston, where the 4 lanes choke down to 2 lanes, same for the freeways going NW and W
4) most of this is going according to plans, we are still 36 hours out from the end of practical evacuation, it is just going to take a long time
5) if you open up the inbound lanes closer within the city, you simply move the traffic jam a few miles north, because instead of 4 lanes going to 2 and 2, you'd have 4 and 4 lanes going to 2 and 2 at the choke points
6) contracted fuel trucks are mobilizing to refuel those who run out of gas on the side of the road
Exactly. Looks like they are doing about as well as could be expected.