I'm sure the decision to offer only a Chama/Cumbres turn was not taken lightly but based upon observed daily ridership patterns. There has been considerable evidence over the past decade that a goodly number of riders are already in the mountains on vacation when they decide to ride the C&TS -- just one more thing to do while on vacation. Although the numbers vary a lot, on average nearly half the riders on a given day only make reservations within two days or less of the day they ride (including "walk-ups" that morning).
So many of these folks are already enjoying a week of vacation and come Sunday they are often heading for home. The exceptions are the Sundays that fall in the middle of three-day holidays and Sundays in fall color season that attract day-trippers.
Given these rough patterns, the new Sunday schedule is a worthwhile experiment. Time will tell.