Because quick, short passes don't work against press coverages and packed defenses.
You may have noticed that a lot of quick, short routes got jumped by opposing defensive backs, leading to pass deflections, interceptions, and some pick-6s. That happens when you throw quick, short passes to receivers who are well covered. It gets even tougher when your QB can't read coverages or when your receivers can't run accurate routes on pace.
Opposing DCs are smart. That's why they are SEC coordinators. They look at film and quickly see that UK's OL can't block blitzes or quick edge rushers. So they press our receivers and rush our QB with tactics they have seen on film from our previous games. This makes them vulnerable to a hitch and go route, but only if our OL gives our QB enough time to find the receiver and the QB can actually hit the receiver.
We have just summed up why Stoops and Hamdan are replacing their QB, WRs, and almost all of their OL. Stoops says he can fix this. I believe he is sincere. We will see if he can actually put a productive SEC OL on the field, because that is a prerequisite. On the whole, I like the changes so far. Stellato and Dowdell are big gets. Calzada, unlike Vandagriff, has a high floor. But we still need a new SEC caliber LT, and it would be nice to snag one more big, athletic receiver. There is still time.