Well, you seem to be comparing rank-and-files here, so, leaving out guys like Magos and such, your average techpriest is still fairly squishy, and a servo-harness is, I would expect, rather heavy. Even Astartes, who are human tanks, need power armor to use one. It's not that a techpriest couldn't make or get his hands on a servo harness, but rather that it may well weigh more than he does and he wouldn't be able to move with it. As to the conversion beamers, Astartes chapters are, as a general rule, really good at archiving things and taking care of old stuff. Consider the Dark Angels: Not only do they know about the Heresy time period, which much of the Imperium doesn't remember, but they even have a comprehensive list of legion members who turned traitor. To link it into the question at hand, they don't have CBs because they make them, they have them because they've kept them working for the last ten millenia.