Because the head of CDC believes THIS: “We’re not going to hermetically seal the borders of the US. We’re reliant and interdependent with the world for travel, for trade, for the economy, for our families and communities.”
Or, becaue the CDC lacks jurisdiction over which flights can land in London.
If (and it's a big if) the powers that be at CDC concluded that West African airports had to be closed, the UK, Canada, Australia, and NZ would make the announcement the same day.
Unfortunately, the powers that be in all of our countries believe that quarantine and isolation, applied on a global scale, constitute "discrimination" which, in their minds, is equal to racism.
Therefore, the planes will fly.
No matter what.