Whats stopping Imperium R&D is a mix of multiple things ranging from Byzantine beurocracy so advanced the Romans are calling it inefficient to the fact that if you build the gun wrong it gets possessed by a literal Daemon. Also the space marines are generally getting the best shit, its the guardsmen who are getting the lower grade shit.

Also the Mechanicus is building new tech its just slow as balls and usually objectively worse than golden age tech which is capable of fucking with time and using the anomolies to annihilate an enemy ship. And the ship that does that may not even be a military vessal by design.