Nothing to do with flying........follow the money !
Harry, your problem is that you are not worth putting through a type rating because you don't bring enough money with you, a 19 year old cadet will have to pay for the ATPL training (£100K ) and then the Jet type rating (£30K).
With this much money at stake you can buy a lot of HR numpty recruitment speak to tell the directors and airline HR departments why turboprop guys can't fly jets.
Harry you just have to face the fact that with your experience you will go through a jet type rating for very little cost, but this cost is not enough to support the hanggers on in the industry who are all getting a small percentage of what the cadets spend to become jet pilots.
The bottom line is money not skill talks, and Harry you don't bring enough money to the party.