Have you noticed how there aren't any Jewish holidays during the summer?
There's only one major Jewish observance -- Tisha B'Av, the day the second temple was destroyed -- and a minor fast, Tsom Tammuz, commemorating the breach of the walls of Jerusalem before the destruction. Both are sad occasions.
We aren't religious scholars (we sell t-shirts) but if we had to guess, it's good long-term planning by our sages.
They knew Jewish kids would be off to camp or Israel during the summer, so they planned the big holidays for the rest of the year.
Bonus for kids: all the major Jewish holidays happen during the school year. Plenty of excuses to skip class and celebrate with friends.
Or maybe it's just coincidence.
Either way, enjoy your summer!