At least it may be dubbed now.
The Brock/Dawn specials were never dubbed simply because Ash wasn't in it. Which I find strange, given all the other pokemon specials were dubbed.
The reason why the episodes aren't dubbed is unknown.
However, if I have to give a answer to you (and not only Cyber) is that answer:
-The dubs buys 48-52 episodes every year to broadcast in TPCi territory (who is eveything but Japan, Korea and China that airs Pokemon).
-Those episodes were in Japan DP episodes. When those episodes are made, the dub had already closed the pack of DP episodes and they are entering the Black and White episodes.
If the dub wants to air those episodes on TV, there are only two ways: Or buying the specials alone; or buying the specials with all the BW episodes. Many TV aren't airing a pack who includes 2 episodes, and TPCi (for obvious reasons, remember BW1) don't want to putt the pack inside the BW episodes because the episodes could air in many stations around the world as a "BW episode" whoich confuse the audience.
THe only realiable way to shwo the epsiodes were through the official website like the B2W2 trailer, the Ranger specials, the Meloetta short, ...
However, because the epsiode only air in Japan, we have consequences of this: The writers will evade mentioning everything that happened in those episodes as much as possible. This is 99% why Dawn didn't mention what she as doing during her cameo. And this is why the only things that it was shown was Quilava because it's unavoidable.
Remember that the writers will try to delete all the possible mentions of an episode a) if the episode hasn't aired (Plasma vs Rockets) or b) if the episodes hasn't aired outside Japan (DP specials) if there'0s enough time (whoich in both case are).
The dub issue is the main reason of why specials of past characters are rare in the anime nowadays.
And the reason of why they do those specials is because a) past characters can't make cameos (in BW1, remember) and they didn't know when they can, and b) because they didn't have time to do it when DP ended for time constraint.
I was a bit off-topic but I had to answer this.
About the prologue of the Mewtwo movie they could air in the offical website days before the movie premiers in the US. I think that this things will be dubbed and it will appear in the DVD. However, I doubt that this would air in the TV broadcast.