Because they seen more from Purdy at the time. So came down to a rookie contract and a 7th rounder at that vs. a 1st rounder and a big cap hit looming ahead.
As even they truly did not know what Trey could do yet.
Business decision / salary cap implications.
However our fans can't understand that, so just hate on players and decisions.
Some Dallas fans want failure here so they can stay in a state of constant whining.
I think Shanahan like that Purdy "stayed on the reservation" much better vrs Lance
(As Bill Parcels said about a young Romo)
Once Lance got hurt, and as Purdy showed well, availability became a higher priority with the weapons S.f. has.
Some QB can ad lib, run, and stay healthy...some seem jinxed