Couple reasons.

First, fuel boils at 100 deg F. This was not so back in the day.

The only place fuel usually boils is in the bowls, because here it is not under pressure.

During heat soak, the fuel evaporates, and sometimes pushes out of the bowls into the motor, causing a flooding condition momentarily.

The extended cranking is to refill the bowls.

I have found wood spacers, and a cool running motor to comat this pretty well. I am also a proponent of electric fans, which keep good airflow over the motor at low speeds.