Of course it's just speculation, but I guess there are three possible answers:
1) they wanted a last 'hooray' , a farewell present if you want', and forgot that this might create issues in PvP (Shut up, PvP guy!);;
2) they are a bunch of dicks that were fully aware of the issues htis might create but they feed on the misery of their moneyslaves;
3) Cataclysm was a rush job, and some/many/take your pick of the pieces just didn't fall together.
Personally, I'm going with #3.
My reason for this is that there are signs of Cata being a rushjob release all over the place (zones like Alterac and Arathi Highlands, the female Worgen , Quests like the Perenlode Tiara that make little sense with the current zone levels, the Maraudon Dungeon Quests, the Weather-Beaten Fishing Hat & poles not being adapted to the new minimumlevel of the Fishing Dailies etc. but no doubt everybody has their personal 'favorites').
Also, I know from a fact that if you e.g. had the old gathering Quests from Steelgrip's ('Stocking up' that required you to get Bearhides, the Ammo quest that required you to loot the still-in game crate near the Wendigo den for those gunners early on the road to Kharanos - sorry I'm just terrible with names) in your log before the Shattering, you could collect and turn in those Quests post-Cata release. This would suggest that there is more 'buried' content they didn't fully remove for some reason - raising the question why it wasn't removed.
Last but not least, we had the weird phenomenon between the pre-Cata patch that introduced the accursed new Talent system & removal of skill ranks, and The Shattering of e.g Dungeon-related gear having radically different stats for that interval only, often with quite a load of Stam (e.g. the Belt and Robe of Arugal and the Feline Mantle, the Razorfen Kraul items and Quest reward Shield etc.) which made sense in conjunction with the increased damage output . Now I may have missed them giving the reason for this, but it somehow makes little sense to me they overhauled a whole cabungle of items just for a few weeks - to me it suggests that till very late in the Cata process they had no idea wether e.g. SFK would stay as a 18-22 Dungeon itemwise.
So it may very well be that originally the Quest would have gone over to post-Shattering Azeroth but it didn't make the final cut.