Quite simply its not in a TOC▸ franchise or management contract with DfT» to provide these additional services; they have a minimum service obligation to meet. ...
Agreed. But doesn't the minimum service obligation require the TOC to provide staff and rolling stock to run the trains they have in their published time table, all be it subject to alteration 8 (or is it 12) weeks ahead because of things like engineering works and - perhaps - major events? Closing a line because they don't have enough crew, and doing so at a few hours notice, strikes me as being way below the minimum that should be provided. It also does no good for their business which should be about giving customers a service they can plan to use and then rely on.