There's over 3 million downloads per month for the official PHP image. That may certainly not be "most PHP users". But still is a a lot of users.
I'm quite certain that your exact sentiment is why the images weren't updated immediately: I'm afraid PHP foundation doesn't see docker images as a particularly important distribution system.
> I'm quite certain that your exact sentiment is why the images weren't updated immediately:
I wouldn't be that certain.
I think it's likely that the vast majority of PHP Docker image downloads are for local development, for one thing.
But nevertheless, it would not come close to the number of websites running PHP from the Ubuntu Server packages. It is absolutely right that the effort was focussed on the distribution route. Because PHP is not "expert" hosting territory; it is still more often whole cloud VMs running the distro packages.