To keep the players, they need to shuffle up what pokemon appear in an area to keep the fun of wondering what they will find next.
My area only has weedle, pidgy, rattata, and venonat with the occasional other common pokemon.
I can go several days and only see the 4 I named...
Heck, zubats and ghastly are rare for me.
They just did that yesterday. The zubats, pidgey X, rattattas are super common everywhere but if you had a dratini nest at your local park it is now evee. Another park that had charmanders is now squirtle. The park right by me was a jynx nest, now it is a scyther nest.
They shifted things to the basic pokemon in the pokedex that appears right before it.