simply because Win7 was a business OS with many people using it - while Win8 and 8.1 were mostly on Tablets and Laptops for Personal-Users with only 3% of market-share
the way Win10 talks to hardware is vastly different than Win8 or 8.1 do
btw - in my opinion AMD should ONLY supply new drivers for Win10 (last 3 versions)
also: Win10 is pretty good now; like we have it on +50 PCs and i have less work as Admin than with Win7 ;)
conclusion: if you still use Win8.1 you should buy Win10 Pro and be happy with newest drivers // have in mind that if you have an AMD APU check if your APU iGPU is GCN - otherwise it wouldnt make sense to upgrade to Win10; as last driver for non-gcn is 16.2.1
PC: R7 2700X @PBO + RX 580 4G (1500MHz/2000MHz CL16) + 32G DDR4-3200CL14 + 144hz 1ms FS P + 75hz 1ms FS
Laptop: R5 2500U @30W + RX 560X (1400MHz/1500MHz) + 16G DDR4-2400CL16 + 120Hz 3ms FS