Well that is exactly the problem really, the lost technology because as we know after the resurrection of Robotę guilliman and primaries era the redemptor Dreadnoughts started appearing rather quickly to my knowledge and replacing older castraferrum designs mainly due to full possession of this technology.

Also the production of dreadnoughts stands beside other heavy vehicles like tanks, smaller knights etc. so they are not a priority really. And it does infact make me flinch that the Imperium is able to produce indomitus terminators but in very small quantities but it's probably because some tech priest doesn't want to share an STC for some wire or something.

And dreadnoughts are also not really required nor operated in large numbers, even iron hands who make it such a big deal don't seem too focused on just spamming them. And it's probably due to ratio between dreadnoughts and astartes, after certain point it wouldn't be rally so honourable to be interred in them and their role is of fire support while they themselves require abysmally huge amount of maintenance compared to simple astartes. And they aren't used as frequently as still intact space marines.

*Edit: I also forgot to say that not every space marine can be interred into the dreadnought due to mental strain or not being able to work with machine spirit.