Warner has a post about why he likes Maven. He might not know it, but he's actually ripping on AppFuse, its directory structure, and build file. I like getting ripped on, so that doesn't bother me. What bother's me is that Warner has comments turned off so no one can get him back.
The main reason that AppFuse uses Ant over Maven is speed. Maven runs much slower than Ant. Period. Also, with an open source project like AppFuse - I try to appeal to the larger audience, who likely has Ant installed. Other OS projects I work on (displaytag and struts-menu) both use Maven and people have a lot harder time trying to build from source b/c of Maven issues. Lastly, I like having a complete download - rather than download-dependencies-after-you-download-the-project like Maven does. I realize if I did use Maven I could package the dependencies in the app - which is likely what I'd do anyway since the main repositories seem to be constantly out-of-date.
Recently, I had a similar experience to Warner. As part of my current contract, I was tasked to write a couple of Maven sample apps. Warner came to my rescue and helped me out a lot, but I felt like I was jumping through a lot of hoops to do simple stuff that was already done in the Ant version of my app. I guess I'm just not a Maven guy. A project that's done right, regardless of if it's done with Ant or Maven, should build by typing "ant" or "maven" - or at least provide you help on what you need to type. Some projects, like Spring and Struts, actually allow you to use either one out-of-the-box. That's a pretty cool idea and likely keeps everyone happy.
It sounds like Warner has re-worked AppFuse to work with Maven. Care to donate your couple hours of work? I wouldn't use it personally, but there has been interest in a Maven version. Some folks seem to like slow build tools.