Indians were never good at scientific method to begin with. Scientific method aka challenging/testing/advancing existing knowledge by peers and verification using experiments was never popular.
Most of the glory lies in individual brilliance/theories/work that was passed down as facts and absolute truths, which does not help in advancing knowledge. Most of the Indian scientific works survive intact unchanged today because of the lack of challenges/advancing of these works by others.
This is due to the Indian psyche of not challenging ideas/elders and the inherent need to maintain status quo. While the threats of invasion, colonization, destruction of centers of learning like nalanda/taxilla etc are true, the greatest culprit is that Indians rest on past accomplishments instead of moving to the next thing and advancing what is already known.
Indians sucked at scientific method of any kind, so development of science never happened beyond the initial work of some savant in their respective fields.