Because the sacred Host IS his Body; it is not his Blood, Soul, or Divinity. But since where his Body is, there necessarily must be his Blood, Soul and Divinity, they are necessarily present by concomitance. Otherwise, a Body without Blood or Soul is a dead body, and we confess Jesus as alive as he is now. The substance (the what-ness) is that of his Body, the rest are present by concomitance.
The wine is changed into the Blood only. It is not his Body, Soul, or Divinity. The substance is that of his Blood. But because Jesus is alive, his Body must necessarily be where is Blood is, as should his Soul and Divinity, again, by concomitance.