"The short story is that the current version of Voice Command relies on microphones that sample at 16 kHz, but BTh headsets only do 8kHz.".
But the real explanation comes down in the blog post. The whole thing comes down to assumptions made by developers when planning software that will take a long time to be written. Decisions have to be made, and sometimes they branch our to the wrong path.
Apparently "the Voice Command team. [They] set out to make the best voice recognition software they could. And, their original analysis said that they could do much better recognition if they designed their database around the assumption that audio would be sampled at 16 kHz instead of 8.
At this point in history, BTh's future was murky. Although it had strong backers, it wasn't doing very well. There were people who promised that it would align the planets, create spiritual harmony, and bring about world peace. There were others who felt that it was a flash in the pan that wouldn't go anywhere. And there were a million more opinions that covered pretty much everything in between. Publicly, Microsoft was pretty cold on BTh at the start, though a number of people in the company were strong proponents.
So the Voice Command team had a tough decision to make. Should they assume BTh would continue to flounder and write a better recognizer that relied on built in microphones (which can sample at 16 kHz)? Or should they assume BTh would take off and everyone would want to do Voice Command over BTh headsets?
They decided that BTh would succeed, but that it was going to take a while to do it. So they chose to go with the better recognizer first. Then BTh took off more quickly than they expected.
There are much more on the post, worth reading it. It does sound as a mea culpa, which is good - but there is no guarantees this kind of decision or assumption won't happen in the future. But every long project needs decisions taken sooner than later so it's understandable.