Why build very similar diesel-electric engines in both AC and DC traction versions?
DC Motors with series field windings have good characteristics for traction and a long history of usage on railways, but the maintenance problem of having to replace the carbon brushes. AC induction motors can only really be used for traction with variable frequency drives, so this is a trade off of simpler mechanics (no brushes) for more complex power electronics.

The rail industry is very conservative, risk averse one because you have to maintain a high level of service 24/7/365 over changing environmental conditions. Although variable frequency drives have been in use on railways since the early 90's, this is still seen as new technology.