There isn't really a "decision" to appeal, per se. The proposal was closed automatically per the system requirements used to determine if you have enough participation to lead up to a healthy site launch. I cannot override that. Re-opening it now would only result in it being closed again automatically.
Unfortunately, this proposal wasn't really "almost reaching the required target". It was a good effort, but reading the Minimum number of people for a successful proposal, you need about 300-400 avid users (minimum!) to hope for a healthy site launch. The proposal author claimed "60,000 fans" ready to build this proposal in Facebook, but seeing only about 12 followers in the last month suggests this proposal didn't have the support reported.
If you have access to a healthy and enthusiastic community wanting to move to this system, I suggest taking some time to assure you have the support of a community ready to move in. To resubmit a proposal, we would need to see a clear demonstration of additional support. Area 51 does not support a process of slowly accreting users over time hoping someday it might just barely make it through. You will need a lot more momentum to lead to a healthy site launch.