This question already has an answer here:
Why can't users cast on-topic/off-topic votes on suggested questions once a site proposal has reached commitment phase? I'm assuming the rationale is that the definition phase is over, so you want the definition to be set in stone. I can understand that, but I'd suggest that you think about the way it comes across to the user who first gets involved during the commitment phase. For me, it was kind of a buzzkill. I found a proposal I would be interested in (Gaming), and followed it and committed to it, then I learn, basically, that I can't do anything else but refer other people until the site reaches enough critical mass to be created, whenever (or if ever) that happens. All I can really do is wait around for an e-mail.