Because the idea of portability isn't just about being portable. It's also about NOT having to install software on your system that increases aspects that slow it down. Also, it's a very cumbersome and time consuming way of doing things. With several apps I have updating this way is a hassle. Not to mention the many other apps that require manual updates.
I only have 4 things installed, in the traditional sense, on my PC. 3 drivers (Korg Electribe, MiniNova, ASIO4ALL) and a GOG game. Incidentally these are mostly portable too, but once it's beaten I uninstall it cleanly.
I wouldn't think it's too much hassle for Rick (the dev?) to add a third download on the download page for a portable zipped version. I would imagine he already has it permanently installed for testing new releases? So for him to zip that install and add to the website shouldn't add much to his workload. Sorry, I don't mean to sound ungrateful, just that it seems a small task that would help so many people out. 🙂