Houston has had attendance problems all year, and it shows in the Peach Bowl's ticket demand against Florida State.
Back when the College Football Playoff was created, the Group of 5 (conferences other than the ACC, Big Ten, Big XII, Pac-12 and SEC) was guaranteed a spot in one of the New Years Six bowls. This year, that would have meant the Fiesta or the Peach, and since the Fiesta Bowl got stuck with Boise State last year, it was the Peach Bowl's turn to absorb the blow.
That meant the Peach Bowl got Houston, as the Cougars are the highest-ranked Go5 team.