Shimano is just responding to market demand. Why would it offer a Di2 triple if it's not profitable?
IMHO, Shimano XTR Di2 was only offered as a triple to showcase Synchro shifting, and regain the momentum from Sram 1x11. Those who can afford XTR Di2 would typically use 2x11 (for racing) or 1x11 for freeriding. For technical singletracks (or even moderately challenging terrain), a triple drivetrain is not desirable because of the potential for mis-shifts or dropped chain. Most mtn bikers can spin over a wide range of cadence, and are not obsessed about close-ratio gearing. It's much more important to get reliable downshifts to avoid the dreaded walk up a technical climb. Another reason is that a 1x11 drivetrain allows the rear suspension to be optimized for efficiency and suspension travel. A granny tends to make a full-suspension bike "bob" under hard pedaling, and removing the front derailleur provides room for better pivot locations (eg. Santa Cruz VPP linkage). So Shimano would sell far more XT Di2 in 2x11 or 1x11 than in 3x11.
Most roadies who are willing to pay for Di2 want to avoid triple cranks due to the higher weight and less reliable shifting. These keen roadies are also fit enough that they don't need a triple. They will take spin classes (using Powermeters) over the winter or do group training for century rides. Gravel grinders are a small niche group, and seem to be happy with mechanical 2x11 drivetrain using larger cassettes (34 of 40T). Ask any LBS about the demand for Di2 road triple and the response will be lukewarm at best.