Because BR had sidings full of fully depreciated 90mph mark 1s just sat there for when they were needed. Unfortunately then the accountants took over and saw this stock as a liability as it had no booked work but had to be maintained so it was all sent for scrap to save a few quid.
They also had quite a few freight engines that were not needed at weekends to haul them at a minutes notice as well.
Nowadays there is no spare stock and a lot of maintenance takes place at weekends so that the maximum amount of stock is available during the week meaning there is less stock is available. I wont mention the stupid way running trains is costed these days!![]()
Thanks.