Because the OP wants to visit both the national park and the temples, and you can't sensibly do both in one day? Nikko's a fair hike from Tokyo as it is, it's 4 hours on a train just to get there and back.

Nikko itself is not an onsen town, but Chuzenji about 30 minutes away is, and it's the main gateway to the national park as well. There are a bunch of other hot springs in the vicinity, just stay away from Kinugawa.