I don't believe that SDS has ever came out and said that the reason they don't add more things to franchise mode is because very few play it. That is a theory formulated on the forums.
And while the improvements/additions to franchise mode may not be considered back of the box big time improvements/additions/features, they have made improvements over the years. It's not as if nothing is being done to it.
The addition of critical situations, jumping in and out of games, and PHASES were all done in an attempt to attract more users to the mode. PHASES was added last year to help those who may feel overwhelmed by the intricacies of the baseball season feel more comfortable and be willing to play through a season.
This ideology that SDS is purposely trying to move people away from franchise mode in an attempt to remove it and force all buyers to DD is, imo, preposterous.