the reason they don't do it is because EX hcb +K does more damage and essentially has the exact same function of a free juggle, in HD combo's the EX Zanrestuken keeps the combo going extra long, In DC's there are better options than that move
Looking at the damage values on the Wiki, does EX Hien really do more damage? Does scaling kick in from all the hits or something? Damage on the Wiki makes it sound like a mini-DM.
While good reasons, it's always a shame to see moves (especially moves with as many animation variations) go totally unused by a majority of the players. In one breath, they complain about wanting moves back... and then in play, they don't use all the moves their characters have, lol.
Even the K' players vary between using a (Crow bites -> Blackout -> Crowbites), {Crow -> EX Minute Spikes/Narrow Spikes -> Crow), and (Full air Crow -> EX Air Minute Spikes -> Crow), or some variation that occassionally uses EX Crow Bites (which is basically XI's Heaven's Drive DM).
Ahh well, to quote Mr. KOF from the Robert Forum:
I'd rather try 20 + variations of combos including making use of all his move sets into one combo than going for the most damaging. If i didn't care about style i'd just perform super/drive cancels all day and forget about HD cancels...but you know what? " If I do not glow, it's not the time for you to go"! This is the way i play KOF!
* Is it sad that I rememberd this quote from November of last year? LoL.