I never claimed to be otherwise :P
A web app was out of the question for item sets, since you would have to have the user navigate to their League directory, scavenge around for item sets, and upload them to your website to edit sets. To save a set the user has to go and download a file from your website, navigate to the League Directory, then save the item set in the right folder. The user experience would frankly be terrible if that was how I did it, discouraging people from making/editing sets at all.
Java's the only desktop language that I've managed to do GUI development in (I've fiddled in Python, but with little success). Combine that with its cross platform nature and my experience with it made it decently appealing for the project. Pretty much everything is going to be waiting on user input, so speed isn't an issue, and telling someone "Go download the latest version of Java" usually results in them understanding what to do and getting it right, rather than "Go download the latest version of Python" and hoping they can set up class paths right.
But if you don't want to use a "heretical" tool you're more than welcome to build your own version :)