The answer is "Historic."
SureFire standardised on a 1 inch diameter body right from the beginning - their products fit standard 1 inch diameter 'scope rings'.
Due to the modular approach SureFire pioneered there is a need for batteries to be accessed from both the body and bezel ends of the lights where several tubes are threaded together.
Moreover due to the shock-isolated Lamp Modules used by the WeaponLights the batteries
had to be accessible from the rear of the light because they are physically stopped entering the Lamp Assembly through the Lamp Module Body/Adapter Collar, and sometimes, moreso in the old days there was an off-set adapter between the Lamp Module and the body.
Also in the old days SureFire gave operators the option of a number of remote switches, and combined flag and remote switches. These required space to wire-in. 1" diameter was sufficient. The E-Series diameter is not.
As mentioned it was decided to use a thicker body wall for robustness and the ability to use high quality threads, as well as achieve the 1" standard.
As also mentioned there is a need for tools used by those in adverse conditions, such as stressed conditions, the cold/wet, whilst gloved etc, the tools need to be ergonomic for people, often with 'man-sized' hands to use - large enough to easily grip, operate and retain. The push button pressure switch diameter and profile is no accident - it was evolved from SureFire's development and interaction with its customers.
Note that as time moved on so did SureFire when they introduced their Executive Series (E-Series) with the smaller diameter body. The body being too thin to remove the batteries from the TailCap-end because SureFire wanted to retain the high-quality threads.
Even with the A2/L1/A2 SureFire found that the TailCap-end of the body could only be made so thin - requiring modified threads and a new approach with a inserted contact ring.
Check out Shelby Chan's photo of the cut-away A2:
