I would think that the Heralds have been around since the Tranqualine Halls, which sound pretty nifty. Then they got thrown out, the Oathpact was made with 10 'heroes' (the Heralds), the majority of the population probably died in that desolation or in the poor living conditions due to having to rebuild their whole society on a new planet. Then, after uncounted Desolations, their technology is at the level we see today.
I doubt the Heralds have much time to impart much knowledge except war-tech. They evidently die frequently in the Desolations, and are otherwise taken back relatively quickly after the end of one. The Oathpact may also prevent sharing of knowledge other than what is necessary for survival. It may also be that after the first few desolations, with them actively trying to impart their knowledge on the pre-Desolation societies, and the results being that even more destruction happen somehow, they decide not to to save people or something. Or even that they have forgotten how stuff works other than their deep memories of a better time, cause they're getting tortured by Odium and then thrown into a Desolation every now an again.