Two reasons: because of the technologies available at the time and of course costs. Back in the heyday of type catalogs, art directors would order typography on glass plates (which eventually gave way to film) or cheap methods such as lettraset could be used as well.

People ordered headlines like pizza. "Hello, I'd like to order this phrase in Eurostile at 72 points and 42 picas. You don't have Eurostile, shit. Well what else do you have by Novarese. That guy's hot right now."

Ads like this were created by hand and there were much fewer typefaces available in inexpensive formats. Also, this still goes on today, but every foundry would be chasing the other in terms of licensing type and then offering popular type for cheaper! This is why there are over 27 versions of Garamond, btw.

Now when the computer began taking over with graphic work, obviously there were only a few foundries that knew what the fuck they were doing and so they dominated the industry for awhile.

The brilliant part about all of this that the largest type foundry in the world has around 22 employees. The upper echelon of the type world is very small. To a large degree, type is invisible in our culture, but knowing its history has taught me that if you can identify type by foundry, you can pretty accurately predict the decade of its application.

So yeah. Money and Technology.

Ninja Edit: Come think about it... fashion, too. I used an example of it.

Edit: FUCK, I should've pointed you to what House Industries is doing with all the Photo-Lettering archives they acquired! I got to talk about it with Andy Cruz from House. It's fucking interesting.