Why most of the devices supported by CyanogenMod are from HTC?
Because, for the most part, HTC doesn't try to lock down their phones. Compare this to Motorola: all their phones have locked bootloaders except for the original Droid. However, Motorola has been showing signs that they might be changing this behavior in the future. A lot of the HTC phones can use the Unrevoked method to root their phones (except for maybe the Thunderbolt?).