The Ukrainians seem to be pulling brigades even from Kursk to halt the Pokrovsk collapse.
It is likely they realized that the ongoing Kursk offensive or the planned Zaporozhie offensive will not work, especially since their Western masters have not unlocked their ATACMS and other long-range weapons to be able to strike deep into Russian territory.
It is also likely the Russians will hold off advancing further (at least so rapidly) in the Pokrovsk-Konstantinovka line in the near future, and instead will push exactly where the Ukrainians are pulling troops from.