Promises and history and network effects. Generally you more or less buy into a system of instrumentation. Obviously you can always run wires from terminal A to B across brands, but you can work _much_ more quickly and neatly when working with the standard-for-your-shop stuff. Your shop will have oodles of terminals and buttons and relays in stock, and know exactly where to get the oddball stuff, etc. The resellers and supply houses all keep inventories. If a part does go end-of-life, you'll probably get an email if you've ever purchased that part, advising you to make final purchases for the long term if needed, etc.