When ordering planes, airlines usually have to firm up their decision on configuration, customizations, and some other stuff about a year or more out before the plane is actually delivered. This is largely a supply chain limitation [suppliers have their backlogs for seat orders, specific equipment/tools, etc.], so UA cannot unilaterally change their preferences in the middle of the process.
All planes being delivered this point and on will have Economy Plus [as well as the last handful 739s], it is those that were delivered earlier this year that were delivered without E+. UA is working on 1-2 739 per month adding DTV and E+.
This is a good website to see where the sCO fleet stands in terms of reconfiguration progress:
http://www.aquawise.com/PMCOaircraft.html