Why does Steam not sell the Mac versions of all games? [closed]

What games are sold on the Steam Store is entirely up to the publisher. If there's a Mac version but it isn't on Steam yet you could try to contact the publisher (EA) to see if they'll put it up.

The SteamPlay feature is very new right now, you'll just have to wait a bit for it.

There is also the issue that Mac versions are usually ported by other companies, so they're depending on sales for that version too. Steam makes it impossible to sell Mac and Windows versions of games separately, so a publisher has to think if it's worth it to sell two full-priced versions of the game for the price of one.

UPDATE: Plants vs. Zombies is now available as a SteamPlay game.

Just speculating here, but there might be changes that need to be made to a game to get it to work with Steam. This won't be true of all games so we need to look elsewhere.

As Valve have only just released Steam for the Mac the game makers could be waiting to see if it takes off on the Mac and how much demand there is before committing resources to existing games. Even if there's no coding changes there will be things that need to be done and there is the opportunity cost of not doing the other things those people could be working on.

The bottom line is always profit so they might have decided to concentrate on new games that would give a better return on investment.