This cannot be emphasized enough. The Cubs were able to paper over their rotation deficiencies through trades and FA acquisitions over the last decade, but this team was unable to develop starter quality pitching prospects up until recently, be it due to inability, injury, or luck.
Steele was a middling LHP prospect until he somehow figured things out when they stretched him out (which is why I have *some* hope Wicks can turn it around) and Horton looks legit, but things were barren on the pitching front up until the last few years. Remember Brailyn Marquez and Ryan Jensen? How about Thomas Hatch? Dillon Maples ring any bells?
Things seem to have improved compared to where the system was a few years ago, and I think there's room for optimism in the system right now, but right now those deficiencies are hammering the Cubs' pitching depth.