iirc, and people can correct me if I'm wrong because this is definitely NOT my forte, the problem lies in chip densities. Most sources say they're going with "slow" ram, meaning they're likely going with GDDR3. I think GDDR3 chip densities top out at 2Gb (256MB). So in order to get 2GB of memory, they'd need 8 chips on the board. They can go higher than that, but the problem is using more than 8 chips requires a larger circuit board, there are bandwidth problems, heat problems, and of course cost issues. The same rule applies to GDDR5 and XDR, which is why a lot of people think it's unlikely MS or Sony will go higher than 2GB either. It's not impossible though, and the easiest way to go about breaking the 2GB barrier is by using a split pool of different types of ram, probably a mix of GDDR5 and GDDR3. But it's very unlikely nintendo is going to go that route because of cost, heat, and size issues. It also appears that they're going to try to reduce reliance on ram by incorporating a relatively large amount of EDRAM.
edit: Whoops, I was wrong, I think GDDR3 chip densities top out at 4Gb and GDDR5 and XDR tops out at 2Gb