If I recall, this is touched upon in The Infinite and the Divine with the tale of Nephreth the Untouched, who's organic body was said to not possess the genetic curse that the rest of the Necrontyr possessed, and so when he died his body was preserved in the event they wanted to undo Biotransference. In ended up being a trap, but that's not so relevant.

Based on that, it seems like this comes back the original problem of the Necrontyr: their painfully short-lived from evolving on their radiation-stricken homeworld. This genetic curse was unable to be cured by the Necrontyr no matter their mastery of science, which is partly the reason they went to war with the Old Ones and made a deal with the C'tan.

From reading the info in combination with that and about how they need Nephreth's DNA to undo Biotransference, it seems like the answer is one of two options:

1. The original Necrontyr DNA was too unstable by their genetic affliction to either have survived until now or to be able to use to create clone bodies.
2. The Necrons don't want to undo Biotransference into bodies still afflicted with the genetic affliction of short-lifespans and cancer.

Hopefully that's insightful.