This question makes an assumptions that isn't true - namely that "Wikipedia not want to improve their app for iphones". That's patently not true; we absolutely want to improve the iPhone app experience.
Here's why it hasn't happened: We're poor. It's that simple. We don't have a lot of money to pay large teams of app developers.
But take heart, my friend! I know for a fact that we've spun up a team devoted specifically to this. In fact, I interviewed one of the fellows we hired who will be starting soon.
So be on the look out for updates and changes.