Why has the Bombardier CSeries been delayed in starting service with Delta Air Lines?
This delta news brief from February of this year alludes to the fact they are waiting for the Alabama production facility to be completed, presumably where they will build the aircraft.  


  Delta still intends to take as many deliveries as possible from the
  new Airbus/Bombardier facility in Mobile, Alabama, as soon as that
  facility is up and running. However, as the ITC notes, the airline has
  contractual commitments to begin taking deliveries later this year and
  the ITC decision clears the way for Delta to accept deliveries in
  Canada as well....
  
  ....and while pleased that imports from Canada can now be part of the
  near-term solution, Delta looks forward to taking U.S. manufactured
  aircraft from Mobile when that facility opens.​


This article notes that during an earnings call they mention early 2019 as the projected start of operations for the aircraft. The article also notes they were slated to take 15 aircraft in 2018 but "program delays" prevented this. The article does not detail what those delays were (presumably the lack of the Mobile facility contributed along with the tariff issues).