This is an interesting idea to explore. I would have expected it to be the opposite ("I don't know what to write about on English Wikipedia, since all the main article topics are already posted") (this is obviously not true; I'm just talking about a trend of perception as the encyclopedia grows).
Is it:
- Because ("readily available resources in English (including English Wikipedia) in 2017" - "size of Korean/Czech Wikipedia in 2017") is a bigger difference than ("readily available resources in English in 2001" - "almost empty English Wikipedia in 2001") and people speak English well enough they don't mind using it? This suggests people might contribute more in their native tongue if they had a particular motive, such as:
- Sharing knowledge with people who are not as proficient in English
- Cultural autonomy
- There are more free online resources in Korean/Czech in 2017 then there were in English in 2001 (thus, less perceived need for a Wikipedia in that language)?
- Cultural?
Finally, did you ask if people from these countries are editing English Wikipedia in addition to reading it?