It's a better season for logistics; the rainy season hasn't set in yet, and this last winter was mild enough that the ground wasn't frozen in March; it was muddy for the Russian offensive. This ground is dry and solid. now.
And -- my thinking is that the Ukrainians have a lighter logistical tail than the Russians due to interior lines. Combine that with nice, solid ground, and weakened, poorly-motivated Russian forces manning very long defensive lines. It won't be easy, and probably not completely successful, but I think the Ukrainians are doing the right thing by racing the autumn rains and pegging out claims before the weather shuts down mobile warfare for a few months.