Worgen DK's are way more likely to exist then pandaran DKs, simply by virtue of the fact that the Lich King had Arugal working directly for him in grizzly hills for however long he was alive again, before we killed him (again). There would have been a vastly larger supply of potential Worgen for him to convert then pandaran.
I mean, Pandaran adventurers out and about in Azeroth in general were rare-er then hens teeth. The very idea that there would have been even remotely close to enough of them anywhere near northrend or during the third war to allow for the inclusion of their race in the DK ranks pre-cataclysm is rediculous, to say the least. There might, lore wise, be enough pandaran to meet those criterie for maybe one or two NPCs of the race to be DKs raised under Arthas, but that I about as close as you could get, and that would REALLY be stretching it. Allowing them to be players means the lore has to account for there being dozens or more of them all running around at the same time (since it is not feasable for you to be the ONLY "insert race X" DK out there based solely on you being a player). Lore wise, it simply would not work.
---------- Post added 2013-05-11 at 12:49 PM ----------
Pretty sure your background, either racially or professionally has absolutely nothing to do with you being raised a DK. Remember, ALL DK's were SOMETHING before being raised as one. It is entirely possible that the person who is now a DK was a Druid or Shaman before his death.
You have been raised from the dead and empowered by Arthas, the only thing remaining of your old life is some of your memories, your core personality, and that is about it.
In all honesty, I am more surprised that they allowed Draenei to be DKs, considering they are A: not native to azeroth, and B: only one step removed from basicly being demons. An undead Demon sounds rather bizzare doesnt it?
Then again, DKs are not really true "undead" as i undersand it, but something halfway there (sort of like a mix between Necromancy and Really powerfull mind controll).
I didn't say lorewise, they couldn't be one, I meant it is just the way Blizz wants to portray Pandaren as the true neutral race, and by making Pandaren to be part of a class that could be either deemed good or evil would diminish the concept of Pandaren; Neutral, Balance, Harmony.