I'm sure there were "some" people who didn't need to travel yesterday. But I suspect the vast majority felt they didn't have that option.
Last year, I booked a Wembley stadium tour as a consolation treat for my son (due to cancellation of a big family holiday due to my wife's cancer!). We pre-booked advance tickets, pre-booked a hotel, pre-booked the wembley stadium tour, both of the latter, being non cancellable, non refundable. The night before, Northern apparently cancelled the first train of the day (Sunday) meaning we couldn't get to Preston to catch the London Virgin train. I'm sure railway workers would just shrug it off and glibly tell you to simply go another day. But that costs money, another day off work, an unhappy son, and re-arranging everything again.
There won't be many people going to London on a whim, with nothing booked, just to do some random shopping or sight seeing, where they can easily cancel at the last minute and go another day with no consequences. Most people will have reasons, commitments, pre-bookings, appointments, connections to airports for holidays, etc., that just can't glibly be changed.