Because any resource which is thought to be safe from bombardment will be utilized. Sadly this is extremely common all over the world. In war zones, armed forces use hospitals, schools, etc, as a base of operation (which invalidates its protected status by international law), then cry fowl when their cover is eventually bombed. Not saying Ukraine used Chernobyl for military purposes, but it would definitely not surprise me if they did, or if Russia wanted to rule this out. Either way, war is awful, too bad we are only going towards more war and more conflict. We're a dumb species.