It's true that it has always been hotter in Florida during the summer, but recent years have been worse. And people who are experiencing that are deciding they don't want a repeat, so they are choosing to travel to Florida during the cooler months (myself included), which is why previously slow weeks in November, January, February etc are seeing MANY more guests than they were in the past and people are coming away from those trips saying "It was a lot more crowded than I expected but the weather was great!" For me, and many others, comfortable temperatures have taken priority over the length of the lines.... and I'm not overthinking that.
It might be "hot" everywhere in the summer... but in California it is in the 70s right now with low humidity. HUGE difference from Florida... and there are awesome Disney parks there too.
It really isn't that hard to imagine why summer attendance is low at WDW.