How is this proposal different from existing stack exchange sites, particulary StackOverflow, but also sites like Cross Validated and Signal Processing? The Area 51 FAQ explicitly states that sites that fall under ones or more existing sites will not be accepted, and numerous proposals have been rejected for exactly this reason.
There are numerous languages that are both more popular and less popular than MATLAB that do not get their own site for this reason. So just popularity, or lack thereof, is not justification.
Simply stating that some other software, such as Mathematica, got its own site is not sufficient. Mathematica got its site because the people behind the proposal were able to explicitly justify the site as not falling under any existing site. So people behind this proposal will need to do the same if the proposal is going to succeed.
I asked this question on the last 2 MATLAB proposals but did not get a satisfactory answer. If you would like this proposal to fare better than they did, you really need to be able to justify its existence.