They are blocked because they had no more place in disc 4 with the very long cinematic ending, so they decided to cut on the towns. To explain that in the game, they did that Ultimecia's power blocked all the towns.
That's what I've always thought myself.
Originally Posted by Pharoh Amon Khan III
But I will also say/add this: I thought it was because at "Disc 4", "we" were in a World at the intitial stages of Time Compression that the towns were "Out of Sync" with the current time of Ultimeica... But that's just my assumption. Look to previous post by Sir Bahamut for his view on this subject matter.
And that sounds like a really good excuse that they can come up with to explain it away.