because the carriers want it locked down because a lot of dumbasses its true mess up their device when installing wrong ROMs and kernels. run to ATT and say the phone doesn't work omg I hate HTC. So ATT exchanges it then finds out the phone was rooted and unlocked and caused by user error.
This is one of the greatest things HTC could have done, now most people don't have to be stuck with Sense if they don't want to be. It also appears that the 64GB may be At&t exclusive, so the Dev Edition is a lifesaver for some T-Mobile customers.