It was suggest years ago by Crossrail that it should be allowed to work collaboratively with HS2 in regards to workforce and skills training, this was ruled out by the Government as being uncompetative, increase costs and add delay.
The other thing not mentioned is the Brexit effect, large numbers of EU countries skilled workers migrated back to the EU as there was no certainty they and their families could remain. For a large number of the jobs in Crossrail do not fit into the UK Governments essential skill profile