Here is what I found, its quite interesting, I hope it helps..
https://www.fbi.gov/services/laboratory/biometric-analysis/codis/codis-and-ndis-fact-sheet
Photosynthesis is the answer
Trichomes increase the chances of survival for plants living in areas with infrequent rainfall.
If a plant in a very rainy place had trichomes, it would probably adapt and remove them as time passed as they are unnecessary.
