Poly was always popular even pre-COVID, but availability has been particularly low/extremely limited since the hotel side reopened July 2021. Whether it's the seemingly very popular room renovations, continued placeholder travel for the future/rolling over of cancelled trips, or Disney's booking algorithms, who really knows. I could say the same about many Disney resorts, but Poly seems to be among the most commonly mentioned as having very poor availability compared to past norms.
That said, keep checking - people are changing plans all of the time. If you have the flexibility, it can also help to tinker with your check-in date and number of nights just to see.
Hope that helps. Good luck.