Ultimate (1.4.7) is locked, there will never be another update to it because it's not possible to update without breaking every world. Hence the 1.5.2 packs were made. Because of MAJOR changes to mods (thinking TE, IC2 for starters) it's highly unlikely Unleashed or Unhinged will be upgradable to 1.6.4 without something drastically breaking, not to mention the biome adjustments made by Mojang to 1.6 which a 1.5 map may or may not be able to cope with.
It's not FTBs fault, it's not Mojangs fault, it's not the modders fault, the game is in an evolution of an ever growing state (by Mojang) and the modders are getting more inventive with every new mod bringing things which we as players certainly didn't think was possible (think EnderIO pipes and AE storage).
Sometimes the old cannot adapt to the new (maps) and rather than force it and break it, everyone is better starting again IF THEY want new stuff.