Victorinox is one of, if not the, largest knife companies in the world, which has been selling slipjoint non-locking knives for like 125 years. Wonder how popular they are? Eh, I'm sure they don't sell many knives. Heh, just kidding. Whoever your favorite knife company is, they would absolutely murder an entire truckload of puppies to have sales figures approaching even the same stratosphere as Victorinox.
Also, slipjoints aren't popular? I wish that were actually true, as I'm sure the rest of Great Eastern Cutlery's fans do as well. Oh, and in before folks who love Case knives (another company that sells hundreds and hundreds of thousands of knives a year) chime in.
Knife locks are to protect people who were doing something dumb with their knives. People got along without locking folding knives blades being in broad circulation for hundreds of years. It's only a recent phenomenon where it's become so important, and that's because a lot of people are ignorant and seem not to have ever been taught to use the right tool for the job.