I think it’s just historical. Combination pliers are really a British thing, and U.S. makers haven’t traditionally offered them.
Whenever I’ve mentioned this in the past I’ve usually got the same answers - ‘what about lineman’s pliers, or slip-joint pliers?’ Well, lineman’s pliers differ in not having the curved gripping area behind the flats, and slip joint pliers are, well, slip joints.
Some of the time, I think folks get by with lineman’s pliers, which is why you see far more in the U.S. than in the U.K. where they are really a specialist tool. Some of the time folks get by with slip joints, but I always find these seriously frustrating to use when you’re used to combinations. I think some people also tend to use water pump pliers (look at the numbers of Knipex Alligators and suchlike that are sold) when again these are almost specialist tools in the U.K.
Personally, I find that combination pliers are amongst my most used tools. I use them on just about every job and have a selection of sizes available, mostly British made!
Channellock are one of the few U.S. makers who offer them - they’re nice tools.
The Snap On pliers were originally the old Vacuum Grip line. I believe Snap On bought them out years back. They’re good tools, nice and solid, but sadly I’ve noticed a gradual detioriation in quality over the years. The joints have become simpler (cheaper) and the finish rougher since I first bought them in the 90’s. I still quite like them, but at a retail price now about 3 times that of similar British, French or German products, I doubt I’ll be getting more!
None of them match the quality or practicality of the last of the old ‘Vacuum Grip’ style that were still available up to the mid 90’s if memory serves!
The design of combination pliers couldn’t be patented, and I doubt ever has been. They have evolved over the years and I have pairs in my possession owned by my grandfather that are 90 ish years old, and little different to today’s
