Yes, the Zodiac is a more powerful handheld console, but the gp32 is now a few years old, so it was bound to happen.
However, not everyone is going to jump onto that bandwagon. Have you seen the terms and conditions for getting hold of the Zodiac SDK? The GP32 SDK is freely available, as well as all the documentation and hundreds of example code written by other users. The Zodiac SDK is closed, may only be downloaded from authorised websites, requires you to sign a non-disclosure agreement, prohibits you from even disclosing your own source code if it uses there SDK, etc, etc. For this reason alone, the gp32 will continue to thrive.
The Zodiac is a nice piece of kit, but not everyone wants to learn how to use the console from scratch, and not be able to download example code from others. I believe example code (and general source code on it's own) is an incredibly useful learning resource. By prohibiting this, they are holding back the homebrew community and pushing for more commercial software.