a lack of spawn diversity

i have noticed a decrease in spawn diversity in the town where i play. the diversity was never great here, but before gen 2 dropped, and right after that, it was better for sure. a couple of months after gen 2 came out, it seems like almost all uncommon and evolved forms disappeared. now, i'd say 90% of all spaws i encounter are about the same 25 species. this change has really taken a toll on my experience playing the game. just curious to know if anyone else has noticed the same decrease in spawn diversity? i can't for the life of me understand why this change has occured.