iPod games for iPhone

Is there any particular reason why certain games are made for the iPod, and are not brought over to the iPhone / iTouch ?

I think games like Cubis 2 and Zuma would make a killing on the iPhone / iTouch!

i think both of those games are available on iphone. just with a different name/company

there are plenty of marble poppers in the style of Zuma and Luxor (the latter is even on the store), plus I am going to assume Zuma will be up soon as Popcap is bound to port it.

However, I haven't seen a Cubis clone. If you could point me to one you will make my day =]