Interestingly the Russians seem to have begun drawing the very old D1(M1943)152mm howitzers out of stock, equivalent to US M114 155mm, as there are stockpiles for their shells, which are apparently incompatible with the modern 152mm.==
Because they are working and a lot of them in stock. Better spend them now when they still good. New ones will go to stock for next war.
The trouble is that they are short ranged vs modern guns, and so much more vulnerable to counterbattery. But needs must.
If the Russians have new guns available they are surely going to the front. The other big problem seems to be ammo for them. That seems to be the big reason for bringing the D1s out of reserve.