[Coda]

Java certainly doesn't help, but really it's the fact that Minecraft has to deal with WAY more data than most games. It's creating new parts of the world every time you move 16 blocks, and it's keeping as much of it as it can in memory. And instead of having a small, well-known, pre-made set of things to keep track of, it has to do things like check every single water block in range for whether or not it needs to flow, and it has to do that every tick.

It's actually gotten better over time as Mojang has learned how to optimize things and skip doing things that it can tell it doesn't need to do, but that doesn't change the fact that the world itself takes up massive amounts of memory just to keep track of the parts you can see -- most games don't need nearly that much.

(Tip: Turn the render distance down as far as you can so the game doesn't have to keep as much of the world loaded.)