No idea if anyone has made any noise about this. But they have been working on the dream world since Gen 3! Apparently not very successful either. =P
Since gen 3 is debatable, since hints of Munna’s existence have been around since gen 1.
gen 3 and gen 1 take place at the same time. it’s possible they could have been trying to work on it but it wasn’t working because they didn’t know about munna? i dunno.
Sounds like a resource issue then =P Thats why the gen 5 region could do it because they had Munna while Hoenn, an island region, had a much harder time. That is if you want to talk story wise instead of actual development XD. I’m sure they knew about Munna, just not what it could do due to it being such a foreign Pokemon. Unless this me talking crazy.