Why don't video games make it to all consoles?

Say there’s one game. And it’s only available for 2 consoles. Why not make it available to most of the consoles instead of being selective? Wouldn’t the company who sells it make more money?

There are a few games I’ve been dying to play but can’t because I don’t have the console and I’m certainly not willing to buy it just for one game. If it were available for the DS, then I could play it.