Why most of the devices supported by CyanogenMod are from HTC? 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?).