Ok this response is gonna be long but just hang with me,
May 2023 was when I started my college program, WDW not busy when I moved down there and stayed quiet throughout the summer. Then suddenly in September, crowds both in the parks and resorts picked up and stayed that way through October. Quieted down a bit in November, but after Thanksgiving, crowds picked up significantly and stayed that way through the end of my program at January and crowds were like that until May from what I heard and now it seems like this cycle is repeating again.
I simply do think its a shift in when people prefer to travel, especially since the pandemic. Other than 2021, from what I understood every summer has been quiet. Yes, the weather has been hot like its always been, but it is now easier to take time off of work and/or pull kids out of school for a vacation, so people would prefer to travel when its cooler and cheaper. From my understanding, summer is still priced in the upper tier of the dynamic pricing structure, so people would rather just hold off and go when its slightly less expensive.
The weather is absolutely not the main reason why summer crowds are down, but it is a factor. Working at the resorts last summer, I talked to many guests who exclaimed that it was hard for them to stay in the parks when it is as hot and humid as it has been. I think people are holding off so not only that they spend less on an already very expensive vacation, but can at least enjoy their time outside walking around the parks instead of wanting to duck into A/C after 15 minutes in the Florida humidity.
This is just my two sense and observations I've had since living at WDW for most of 2023, working at the resorts, and going to the parks on a weekly basis.