I wonder if it is in reaction to the planned GTR services operating from Ashford - via Maidstone East.
Potentially? Although that feels like a more commercial/competitive decision, so i'd have expected the bidder to come up with that as opposed to the DfT. Other than that though, the only reasons I can think would be either recognition of Ashford as increasingly being a regional hub, or a way to try and relieve high peak loadings on HS1 by offering a cheaper but still not tremendously long journey times (£134.40 vs £161.30 weekly season and a ~70 minute journey instead of 40 minutes? (based off an existing 43 minute journey on a Tonbridge to Charing Cross service and an estimate of ~25 minutes between Ashford and Tonbridge if running fast)).