Because clock speed isn't the only thing that matters. I am not sure if the 821 is any faster clock for clock. But there could be other SoC reasons to want an 821 instead. Power management could be another reason.
It is not downclocked. there are two different versions of the 821.
The one in the Pixel is 5% more energy efficient than the 820 at the same clock speed. They could have used the other and had very slightly better performance for benchmarking with power consumption similar to the 820, but it looks like they felt the battery and thermal efficiency were a better value. With how well the Pixels zip through anything you throw at them, I'm inclined to agree.
Worth noting that the 821 in any event was not intended to be a massive performance upgrade over the 820, but rather just a slight incremental improvement in mid-cycle development.