The Space Marines aren't likely to talk, while the Inquisition are in large part some of the ones who are working to keep the secret.
I can't see to many traders, whether Free or Rogue, travelling in to active warzones either, at least not in anything more than relatively small numbers, same goes with large business cartels. And the artistocracy are not going to be travelling there except as part of the crusade forces, or with colonization efforts, in either case they're not going to be coming back.
While high ranking members of the military will probably know that their resources and manpower will dry up if the secret gets out, and that there will be hell to pay from both the Commissriat and Inquisition.
Remember that the rich and powerful aren't above being murdered in the middle of the night for getting too uppity with thoughts of independence, or other shenanigans. Perhaps the secret does spread a bit too far, and some merchant cartel or noble house gets it in their head to blow the whistle... and then most or all of them are gruesomely anihilated by an assassin of the Eversor temple.
