(I'll keep this simple as I'm a firm believer in KISS). Probably for the same reason Microsoft tries to sell you a new computer with the same breath it states your Dell Inspiron 9400 laptop (for instance), is not compatible with Windows 10. That my friends is: $. That's right, cha-ching, cold coin, cash money! Give me a break! I'm a hardware man. My personal 'older' laptop, which I prefer to newer ones, has Radeon X1400 graphics and is not supposed to be compatible with Windows 7. BUT IT IS! Not only that, its graphics are both fluid and detailed.
Now I 'get it' that Microsoft and Advanced Micro Devices are in the business of selling hardware and/or software. And, I 'get it' that they need to sell enough product to operate in the green. But...Why do you think Microsoft is giving away Windows 10 pro for a 'limited' time? And, why is AMD stumbling in efforts to provide a compatible X1400 driver for Windows 10?
The reason is because both companies know X1400 systems already are (basically), compatible with Windows 10. In knowing this, both companies (and others), are acting in collusion to get 'rid of', (I clear my throat), 'older' computers? Computers that are not only easily serviceable but are built with better, higher quality components than today's 'laptops' are. (BTW, I'm not talking touch screens here).
So, how about those Radeon X1400 drivers compatible with Windows 10 for the 'commoners'.