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?).
In the past, HTC phones have been the easiest to unlock, which directly translates to their hackability and therefore support from community developers.
Allegedly, newer HTC devices are a lot more restricted, which will put a damper on custom ROM availability. I believe Samsung will soon take the lead in the "most hackable" category, and we'll start seeing more of their handsets on the CM's "supported" list.