This is often the case when steam locomotives are involved. There are many reasons, such as the line is not certified or licensed for a steam locomotive, the line may not be safe all the way, some of the old bridges, of which there are many, may not be up to snuff and so on.
I would guess that the package is cheaper than travelling from London to York separately so it's probably not an option.
I hope you have a lovely time. (Absenthusband is also a great steam buff and waxes lyrical about a number of steam locomotives that he has seen or has travelled behind – or even watched being restored or, in one instance, built from scratch – since he was a little boy in the town where it all started.)