There is a phenomenon called the diurnal convective cycle, leading to nighttime being more favorable for convection in storms.

I was unable to find a definitive answer to what exactly it is, so the following is my own hypothesis from what I did find and what I've learned about tropical meteorology through the years...so perhaps take it with a grain of salt.

At night, you have less solar radiation (insolation) reaching the top of the atmosphere from the sun...well, to tell the truth, no solar radiation reaching the top of the atmosphere from the sun. This results in a cooler atmosphere as a whole. While the temperatures in the upper troposphere are always cold to begin with, when there is less insolation, the overall temperatures are going to be colder. Thus, you see a minimum in the mid-to-late afternoon, when the amount of insolation through the day is at its peak (around the time you see the daily high temperature at the surface), whereas you see a maximum in the overnight hours when the atmosphere has had time to sufficiently cool (around the time you see the daily low temperature at the surface).

If someone can provide a better or more correct/applicable explanation for the topic though, I'm all ears.