Take a look at this excerpt from a study done at Harvard that contents clear skies on the Fourth of July create life-long Republicans:
We ﬁnd that days without rain on Fourth of July in childhood have lifelong effects. In particular, they shift adult views and behavior in favor of the Republicans and increase later-life political participation Our estimates are signiﬁcant: one Fourth of July without rain before age 18 raises the likelihood of identifying as a Republican by 2 percent and voting for the Republican candidate by 4 percent. It also increases voter turnout by 0.9 percent and boosts political campaign contributions by 3 percent. Taken together, the evidence suggests that important childhood events can have persistent effects on political beliefs and participation and that Fourth of July celebrations in the US affect the nation’s political landscape.So it's probably not rain that's the driving force here. The study simply finds exposure to traditional American observances tends to reinforce traditional American values.
It appears the left has long understood this principle. This explains why the left is so tireless in its quest to water down America's traditional holidays.