I'm gonna ask everyone to start thinking for a second while I spin a tale:
"You run to the store, eager to get a copy of Pokemon ______, the greatest game of all Pokemon's history by most critics. It features all 4 regions and Sevii Islands, maybe a small Orange Islands Region with all the Gyms, all 493 pokemon without needing events, 32 gyms, 3 Elite 4's(Johto and Kanto share one, remember?), about 15 HM's, and every single seemingly-useless side event from every region. Team Rocket, Team Magma, Team Aqua, Team Galactic. Some of the most memorable places of all time! Cerulean Cave, Tin Tower, Whirlpool Islands, Cave of Origin, Mount Coronet. You get a starter from each region. Pokewalker Technology, Contests, Pokeathlon, Bug-Catching Contest, Game Corner, Battle Frontier. All the sudden you start playing and nostalgia bites at you from every direction. Your Pokemon walking beside you, just as it did in Yellow, and HG/SS. This game is critically acclaimed for packing such power into a single cartridge!"

So after hearing the story of the Pokemon _____, which doesn't exist and may not ever, do you wonder why this has yet to happen? G/S/C had two regions on the GBC cartridges, you'd think Game Freak could do this on a much stronger DS cartridge! Now, for the final verdict. If this game existed I'd spend $70+ on it. Would you buy Pokemon ______?

I think I probably would, if I made enough and had the console to play it on. BTW, I believe that Pokemon Black and White let you head back to older regions, but I'm not sure it has quite that many features.

I'm glad they haven't. For one, they would stick it on the Wii so they could pull the full price point of Wii games (which arguably is closer to what a game like this would be worth), losing the connection and portability that Pokemon games thrive on...
But even more importantly: It would be too much. The idea of having them all together means that either the difficulty gets ramped up to the point that it would take insane grinding to get your Pokemon to level 100 so you could actually take on the final E4, or make the first two-maybe even the first three-regions so dumbed down that they're a boring chore to play through. Either way it doesn't entice me.

Well first of all it would probably be for the Wii. Secondly this would be pretty cool but it would take a long time for Nintendo to make it. Also since the storyline would probably be messed up if you can go to all the regions which would also take awhile to complete the game. So I probably wouldn't buy it.