Under the Indult system it was always far easier to get permission for 'one off' Masses than for regular Sunday Masses, and when permission for a Sunday Mass was given, not only was the time sometimes inconvenient (the middle of the afternoon, for example), but a rota of different venues might be involved.
The view seemed to be - and this was sometimes made explicit - that bishops did not wish to see a community of people going mainly or exclusively to the TLM. Why this reasoning was not applied to people who go mainly or exclusively to folk Masses, or the New Mass in Latin, was never properly explained.
The situation changed with the Motu Proprio which not only refers (as the Indults did) to people 'attached to the former liturgical traditions' (ie to people who would like to go mainly or exclusively to the TLM) but actually gives groups (or communities) of such people the right to demand a regular Mass. Far from being a something to be eradicated if possible, communities of people attached to the usus antiquior are now a pastoral phenomenon to be provided for like everyone else.
The main obstacle to increasing the number of Traditional Masses on Sundays today is fitting them into the busy Sunday schedules of parishes and priests, sometimes against a certain amount of lingering opposition. The number is steadily increasing, nevertheless, and the PCED's clarification that parish priests may convert an OF Sunday Mass into an EF Mass on their own initiative will give this process a boost. I intend to blog about the PCED statement separately.