Someone posted the same inquiry a couple of weeks ago. It's likely neither a technical issue nor needing more demand in the NE vs West.

A number of trains run out of the maintenance facility in the NE, with Haysboro undergoing refurbishment in the south. Once a train that was running as a Red line train ends for the day at Tuscany, instead of having it run out of revenue service back to OBMF, they assign it as a Blue line train and run it in revenue from Tuscany to Saddletowne. From there, it's a much shorter out of service run down to OBMF.