Correct, that seems to be the situation.
I believe it has nothing to do with "player development." Player selection highly prizes fastball velocity more than ever, so that is reflected throughout all levels of professional baseball.
Specialization also plays a role - the average number of innings pitched per game is going down, and with more relievers being used, fresh arms can be brought into the game. They know they have to face 3-5 batters, and the most common combination of pitches for a right-handed reliever is fastball/slider, so they throw at max effort and grab some pine.