CJG writes
Presumably, in the morning peak, they think the passenger load would
be too much to bear.

But the evening peak it is? I don't understand that.

Lots of people arriving at Marylebone main line station in the morning
peak will be transferring to the tube - more people than can be safely
handled by using the fixed stairway.

In the afternoon most people are travelling the other way, so the fixed
stairway can safely accommodate all whose who wish to use it.

--
Dave