I think a big part of it is how guns have 'devolved' from a sporting/hunting/self-defense position to how they are seen now (by a large population of owners anyway) is for self-defense only.

I have talked to many people who believe there is no reason to own guns except for self defense.

Given that position ammo capacity is probably the most important requirement for these people (not all - or me however) and semi-autos are the only way to maximize ammo capacity.

Next seems to be the gun's action and simplicity of firing and safeties.

Heck, capacity has even ruled out the 1911 for many as a serious carry gun, along with it's single action design.

This has effectively ruled out revolvers as a consideration for MANY as a primary carry gun however I think like many here, I.E. capacity is NOT an issue and revolvers being as effective as ever for primary carry.

The industry has a lot to do with it as well, promoting semi autos along with high capacity, as them being the 'only' practical consideration as a carry gun.