well the first evidence as the version on WU is usually older than the most recent nVIDIA ones. and we all know how much of a performance increase each nVIDIA update brings as they do mention the performance increase everytime for which games in the change log.
Yes, it's true that for nVidia and AMD GPU drivers, WU usually lags behind what they post on their websites. My guess is that NV and AMD prefer to release it to a relatively smaller, more tech-savvy audience of people who manually download drivers before releasing it to a far larger audience via WU.
But then let's look at Intel's GPU drivers. Much of the time, the WU Intel GPU drivers lag behind their download center drivers. But there have been times when the reverse had happened: an Intel GPU driver appears in WU first and then, weeks later, finally on Intel's website. In one extreme case that I've personally encountered (their G965 GPU drivers), the latest version on Intel's website is
years older than what's found on WU (the version on WU also fixes a brightness control issue that's present if you used the driver from Intel's website).
My point is, none of this is Microsoft's decision. It's not their fault. Nor is it WU's fault. If you don't like that WU's nVidia GPU drivers lag behind what you can download direct from nVidia, you should take your complaint up with whoever at nVidia is in charge of their driver releases, because
that is the person who's responsible for the version difference. Similarly, if I wanted to protest that Intel had basically abandoned the G965 chipset on their website (though not on WU), I should be blaming Intel (though in that case, I really don't care, since WU's more convenient anyway). What you see on WU is what the manufacturer wants you to see, and it, not surprisingly, varies.