Good Pass rushing defensive linemen don't want to play in the 3-4 is my guess. The defensive end is an important position in the 3-4, but it isn't as fun a position as DE in the 4-3. The DE in a 3-4 must be able to fight off blockers as it is more dificult for speed moves to work inside the tackles. Glover, while I think would be a great DE in the 3-4, isn't worth as much to the Texans because they don't need all of his skills. Better to use money elsewhere. Glover probably doesn't want to get smacked around down after down.
Was he always a DT. I haven't seen NO play in a while, but thought he was a DE a couple years ago or maybe he just played like one.
Glover is a DT. If he was on the Texans, he would be our DE, and would fit in nicely at DE. The thing is that we already have Payne and Walker from Jax. Those 2, from what I've read are slated to be our DE's. They both played DT in Jax though. We do need a DT, but Glover is too small to be the NG/DT in a 3-4. Ideally, our DT will be 310+ pounds.