Probably just due to the beta status of the software. If you'll note, most of the other things Nullriver does are commercial ventures. Apptap is, so far, free and it's considered a beta product, so it's not on the main page.
I also suspect that they are waiting to see what the SDK brings before they go to far forward in the iPhone/touch development. If the SDK lets anyone and everyone write and install any app then there's not much of a need for the installer.app product. On the other hand, if the SDK is highly limited or not available to opensource/hobbiest developers then there will still be a need to Jailbreak the touch/iPhone and so they may move it over to their main line of products.
Just my take on the situation.