You pretty much nailed it in your guess. Here are some more guesses.
Rain is one thing, it cools off the land. Also, the difference in temp. between the Gulf and the land is slowly becoming less and less. Right now the Gulf is 78. Last summer the Gulf hit 91 degrees at one point. Insane.

So in the summer, there is little difference b/n the land and the water temps. The seabreeze engine thrives off of temperature difference. The less there is, the less it thrives.

Lastly, the position of the high pressure begins to move more east off the other coast (Bermuda high). In a nutshell, St. Pete sucks for wind starting in about 3 weeks. It was awesome today though!

It will soon be time to start focusing on something other than kiting while in St. Pete (unless you really like torturing yourself praying for something that rarely happens).
Makes me feel sorry for anyone taking lessons right now. They are just going to jones for a few months around here. I know a lot of guys travel out of town during the summer.