Because Russia only deployed 150,000 men while Ukrainian field over 700,000 men. The Ukies are sending human waves in order to overcome the small Russian defenders. Besides, this is a war that the whole of NATO is fighting. In response, Russia triggered partial mobilization, and those are gonna be ready in the Winter, When those 300,000 men join the battle, Ukies will hopefully be done. Let us not forget Ukraine is thrown into darkness. You want to be NATO's cannon fodder, you stay in the darkness is the Russian motto.