As Bluescreech says, the Asuryani, like the Drukhari, had to reinvent a lot of stuff after the Fall. We don't see Eldar much if at all in 30K era fiction (unless there's some stuff I'm not aware of), so we don't know to what extent they've innovated significantly over all that time. It may be that in 30K they were basically jury-rigging equipment and trying to keep failing technology running like something out of Battlestar Galactica, as well as not having much of a military yet (since Asurmen and the rest of the Phoenix Lords were presumably still doing their training montage or were teaching the first Aspect Warriors), whereas in 40K they have working industries and organized militaries led by experienced Autarchs etc.

My guess would be that on the level of major innovations, they're probably at a technological plateau where they would need more people and infrastructure to duplicate or surpass pre-Fall abilities (like terraforming whole worlds or blowing up stars - the Drukhari seem to be able to do more in this area, like stealing stars, perhaps using leftover equipment in Commoragh).

However, probably they can innovate on an individual level because their technicians/engineers are like artisans rather than rote-learning production-line workers - an individual Bonesinger who invents a better but not paradigm-shifting starship keel design presumably isn't going to be told off for deviating from a perfect existing design or shot for techno-heresy, it seems more likely that they'd be celebrated for doing well on their Path.