Because the original design had specifications for 7 round magazines, and many years later in became feasible to stuff an extra round in there to make them 8 round, and also have a reliable magazine.

I only load seven rounds in my magazines, if I need more than 7, the Super .38 holds 9, if that ain't enough then my FNP-40 holds 14.

The eight round magazine concept probably evolved from the competition guys, to get an extra shot before a reload, and now they are in vogue.

 

I remember before double stack 1911s came out we would weld two magazines together, one on top of the other, so we could get a few more rounds before a reload. They were very unreliable.

 

Over the years there have been various attempts at making large capacity magazines for the Model 1911 style pistol. Prior to WWII U.S. Ordnance experimented with a detachable holster/stock and a 20 round magazine, but the attempt was dropped in favor of more pressing military needs. None of the large capacity magazines fed reliably in addition to being very awkward when used with the pistol.

The 8 round magazine seemed to be the final solution of getting an extra round with a flush seated magazine.