Why... would you waste time re developing a program made obsolete by MA3? You squeeze a few pennies out of it but will annoy all your customers when they realize they bought an out of date program and MA3 is where it is at.
Because Marine Aquarium 2.6 "fits" better on the iPhone than Marine Aquarium 3. And it's $0.99, which is a very good price for it.
Marine Aquarium 3 may not be possible on the iPhone because of its use of shaders and large screen width.