Here's why:
* Squirrels are small: Ponchos are designed for humans, and squirrels are much smaller. It would be very difficult to find a poncho that would fit a squirrel, and even if you did, it would likely be too big and cumbersome for them to wear.
* Squirrels are agile: Squirrels are very quick and nimble. They wouldn't stay still long enough for you to put a poncho on them, and even if you did, they'd probably shake it off immediately.
* Squirrels don't need ponchos: Squirrels have their own natural protection from the rain and wind, in the form of their thick fur.
So, while it's fun to imagine a squirrel wearing a poncho, it's probably not something that would actually happen!