One factor may be the TJ effect, so many hard throwers need TJ surgery that you don't want to invest significant resources unless it's an elite prospect - b/c you may be looking at a long development path and leaving the guy exposed in the Rule 5 down the road. It's one thing to wait on a Painter, quite another a McFarlane. So pitching may become bifurcated, take 'em early or quantity late.
The other effect may be the Nola/Ranger factor, half of the top SPs throw 92-94, but these sort of arms tend to be 90-92 at 20-21, so again, low cost later picks and hope one develops v overpaying for a big arm earlier (how many big arms make it after the 2nd rd?). It's similar to loading up with lanky LA pitchers at 17 for $10K and hope one or two fills out.
