Lightning activity day curve - why it has high and narrow peak?
I think the simple answer to your question is that sunlight widens the potential area for storms by producing a better convective environment, whereas nocturnal storms are highly localized based on climatological factors.
While a minority of storms do occur overnight due to local mesoscale features such as nocturnal low-level jets, the majority of storms will occur because of convective forcing. To have a convective environment, you have to have a parcel of air that is warmer than that of the environment it is traveling through, and that is what gives you lift. The easiest way for that to happen is to have parcels rise up from the ground at the peak heating of the day while the ground is much warmer than the air above it.
In an older (and still infinitely useful) thought process of meteorology, there is a temperature known as the &quot;convective temperature&quot; that can be gotten from a Skew-T diagram. It takes into account the dewpoint and vertical temperature profile of the atmosphere and produces a temperature at which the surface level has to get to in order to set off thunderstorms. The best way to get to and beyond that temperature is through the heating of the sun. Additionally, when the air is warmed through heating from the sun it makes the environment ripe for convective initiation through triggers such as fronts.