As the guy above me said, mankind has gone though a regression in terms of technology ever since the Age of Strife screwed everything up. The Emperor improved things when he emerged to lead humanity in the time leading to the Unification Wars and reunited with Mars, but technology was still lacking compared to how it used to be, and STC technology cold never be replicated, only found and reused.
With the Emperors "death", mankind over the next 10000 years gradually regressed even further into an age of religion and ignorance (which ironically is exactly what the Emperor tried to stamp out) where technological advances are few, and further held back by deep-rooted superstitions about technology, where everything (production, maintenance etc) has to be done in specific ritualistic and very restrictive ways to appease the "machine spirits".
Don't get me wrong, technological advances do still exist, but just at a very slow rate... with limited understanding of the more hyper-advanced technologies in existence. Some technologies are only able to be maintained, rather than improved upon. A good example of how important STC technology is was shown in the first Gaunts Ghosts book i've just finished reading (and also mentioned on the Wikia page), where someone told how an STC was recovered that contained designs for a new and improved steel alloy knife that was stronger and sharper than that currently in use by the Imperium. It was instantly rolled out to more than 30 space marine chapters, and the scouts who discovered it were "given world of their own". So yeah, STC's are the holy grail!
I suggest just going to the Warhammer 40k Wikia (it's more in-depth than the Lexicanum) and reading/searching to your hearts content... I can (and have done) spend hours and hours looking through it.
http://warhammer40k.wikia.com/wiki/Warhammer_40k_Wiki
I think 40k is officially my favourite sci-fi setting of all time now... more so than Star Wars even.