Because there's aren't any Astros on the Top 100 Prospect List, except for Jordan Lyles, at #38.
Unlike many of the players on this list, Lyles is not a pitcher who relies solely on raw stuff. He has, essentially, average stuff -- perhaps a tick better than average. But his command and advanced feel for pitching make him a strong bet to be a good No. 3 starter at the big-league level.