Put yourself in the position of a company that's either a start-up, or an existing company just wanting to break into the manufacture of firearms. You want to make guns -- what are you going to make? To make guns, you have to have a design. Hiring a firearms designer/engineer to come up with a new design, one that doesn't violate any existing patents, isn't easy and it certainly isn't cheap.
But wait! There are successful firearms designs out there for which the patents have long since expired. They can be reverse engineered and cloned just for the cost of buying one (can be used), taking it apart, and measuring the parts. That some of these (like the 1911 and the CZ-75) just happen to be some of the most widely used and universally acclaimed firearms in the history of firearms probably doesn't hurt. The fact that the 1911 was designed by a firearms genius AND that the blueprints are readily available on the Internet also doesn't hurt.
So, what would you do? Spend months shopping for a gun designer, wait still more months while he designs you an all-new gun, then build and test prototypes -- all of which costs YOU (and your investors/backers big bucks)? Or do you just pick one of the two most popular guns ever known and jump right into the market?