I suspect it is down to EMR's use of the 'compulsory reservations' flag on their London services this weekend. By falsely marking their services this way in the timetable data (reservations are not compulsory in practice), they prevent journey planners from selling tickets in conjunction with those services unless a reservation can be obtained.
The availability of such reservations can be controlled in the industry reservations system (RARS) based on a number of factors, including the ticket type. Therefore, EMR can set RARS availability up for their services such that a reservation is "available" if the ticket type is an Anytime ticket, but "not available" if it's an Off-Peak ticket, for example.
It's unlikely that they have deliberately set it up to only offer Anytime tickets, but rather they have probably forgotten to add the Thameslink weekend Super Off-Peak ticket types (CBA, CBB) to the list of ticket types for which they offer "availability".
This is hardly a new situation - those ticket types have existed for years - so this is rank incompetence from EMR. It will result in a lot of people being overcharged or unnecessarily delaying themselves thinking they have to stick to Thameslink services. Of course, if EMR didn't abuse the 'compulsory reservations' flag this way, the situation wouldn't arise at all; the problem is entirely self-imposed.
Suffice it to say, you can buy a (Super) Off-Peak ticket by selecting whichever train you like, and then travel on any service - EMR or Thameslink.
