Because they apparently lack the internal compentencies to do so (or, given that they now have a Fields medalist on payroll, it's probably more a case of not feeling any need whatsoever to spend the effort it takes; maths isn't exactly profitable if you can't mine it for PR). Ignoring all the ethical violations they made in the process, the fact that they only ever released slop on the Navier–Stokes problem is part of the debacle.

For example, had they decided to put their paper on arXiv, they would be facing a 1 year ban, followed by not being able to put anything on there without prior peer review.