Why didn't they just widen the tunnels and reuse them for Crossrail,
instead of digging hugely expensive new ones?

I will ignore the geometric aspects of the design of a low speed
passenger less railway vs a modern faster mainline railway.

But from having worked on some early options for tunnelled schemes
through London in the vicinity of the PO railway just how many sewers,
tunnels, etc. are down there. To enlarge the tunnel would bring you
even closer to some of these and require techniques such as grouting
and ground freezing to be used assuming that these are possible as
sufficient clearance may not exist. A 2.75m internal diameter would
need to be extended to around 6m.

OC