The distance has to do with thermal effects bouncing the radio signal off the top of the atmosphere. These effects are much reduced in the daytime. Ionosphere I think.
Some of the big AM stations are allowed to crank up the wattage at night. The west coast has three? radio stations that cover the coast. One in LA, San Francisco and Seattle? KCBS in San Francisco is the oldest commercial radio station. 1909. 50,000 watts. they have to switch to a more directional antenna at night so as not to interfere with the radio station in Toronto!.
Their meteorologist I grew up with is the only radio meteorologist who was banned from the airwaves during WW2 because he was so good it was helping the axis navy and airforce.
Bill D