It became much more important post-Heresy
TLDR: It's mainly because Chaos Space Marines need to steal Loyalist Marine gene-seed to produce new Marines, their gene-seed is too corrupted to produce new Marines the old fashioned way. Loyalist Marines really don't like it when that happens.
If we consider how a gene-seed is created: a gene-seed is implanted in a candidate and assuming the candidate survives long enough, a second gene-seed begins to grow and is then harvested and implanted in another candidate and so on and so on...
There were 18 legions, before setting out on the Great Crusade each legion had many hundreds of thousands of battle brothers, each producing additional gene-seeds.
Additionally, each legions Primarch was alive and an entire legion could be created as long as he was alive.
Once the loyalist Primarchs were dead (missing) and the founding legions were divided into much much smaller chapters, individual gene-seeds became very important. Whereas a legion would have an almost inexhaustible supply, a chapter would not. It's at this point that genetic banks were setup on Terra to produce gene-seed for each new Chapter and each Chapter became very protective over their gene-seed, guarding them ferociously and recovering them from dead Marines at all costs.
Chaos Space Marines cannot produce gene-seed in the same fashion as Loyalist Marines. Their gene-seed becomes corrupted over time due to exposure to Chaos. Their only option is to raid Loyalist Chapter monasteries or scavenge from the dead, thereby increasing even further the value of existing gene-seed.
Some authors probably just like talking about gene-seeds more than others. I guess logically in any post-Heresy battle, especially vs Chaos, Space Marines should always be striving to retrieve their fallen brothers gene-seed. But I'm sure there's many 40k novels that rarely mention them, if at all.