This game is available on the mac app store for the computer, why isnt it available for the iphone??

There is the Big Fish Games version of Virtual Villagers - A New Home available on iTunes for iPhone.

To locate it, simply open the iTunes Store (or the App Store on your device) and do a search for the game.

but aren't the graphics and everything different? And why is it made by big fish games instead of ldw?

You can find a discussion on it here, which has posts by LadyCFII and Arthur about that very thing.