HTC One X Won't Have Unlockable Bootloader
HTC has revealed that the One X Android smartphone, which goes on sale May 6 via AT&T, isn't compatible with its bootloader unlocking tools. HTC provides the tools to users so they may safely unlock their devices in order to install custom ROMs. To-date, it has provided the unlocking tools for 45 of its handsets. With respect to the One X, HTC said, "In some cases ... restrictions prevent certain devices from participating in our bootloader unlocking program." HTC didn't explain the restrictions facing the One X, though. HTC said it remains committed to offering tools for developers and unlocking future devices.
I can explain the restriction
Basically, AT&T said no.