Because they had underestimated the first part of CX season and the impact that would have coming right after his road season. He is making physical leaps every month. He will be a lot stronger next winter than previous winter. If they really think he will be exhausted, because of it, they shouldn't do it obviously, but the Vuelta is the most relaxed GT, and they can pull him out whenever they/he wants. Having him do the Giro as a first GT after CX and hilly classics season, while claiming you want to give him enough rest, would be hilarious tbf.