While they won’t be able to reach Moscow with these, even from within Kursk, I am willing to bet the authorization was withheld on intelligence grounds. A lot of things in the war make sense if you look at it from the perspective that arms support can’t be guaranteed.

Instead of letting Ukraine just burn through weapons, the US and allies have waited until they have the full operational capability to deploy the weapons with extreme effectiveness. They’ve had ATACMS for a while, so I suspect they held off deep strikes until the most effective targets had been identified and the likelihood of effective strike was near certain.