The remake of Final Fantasy III and Final Fantasy IV for the Nintendo DS were positivity received by fans, but a lot of fans want Final Fantasy V and Final Fantasy VI to be released on the Nintendo DS too. However Shinji Hashimoto tweeted that they are still considering remaking them.
"As for FF5 and 6, there are technical problems; presently these are also undecided."
This doesn't mean that the games aren't coming. It might be that Square is waiting for a new version of the Nintendo DS with increased power to be released.