Because optical ADB mice are hard to come by? There are a few, but they're rare.
A good USB implementation might also allow you to hook up storage devices, which would be pretty nice.
There are other (non-DIY) USB-to-ADB projects out there taking shape, too.
But I'm also hoping someone starts making Nubus cards for VGA, USB mass storage, etc. It would just take an FPGA or perhaps even an overclocked teensy.
Because optical ADB mice are hard to come by? There are a few, but they're rare.
A good USB implementation might also allow you to hook up storage devices, which would be pretty nice.