If the stats are to be believed:
-In 2008
18 pitchers in the league could throw over 100 mph and through the whole season they combined threw 196 pitches over 100+ mph
-In 2017
40 pitchers in the league could throw over 100 mph and through the whole season combined for 1,017 pitches over 100+ mph.
HGH comes to mind but they're clearly doping and getting away with it. What do you suppose they're using?
Are you sure it's not training? Similar things with speed and safety regulations in the NHL. They more or less outlawed hard hits and fighting to keep players from getting seriously hurt--namely concussions. Fighting is almost nonexistent at this point but there's more concussions than ever. Because enforcers aren't needed, everybody is fast--fast is the new strong. They're also getting taller and heavier from training. Well, speed and momentum = big fucking hits.
So, back to baseball. Can't dope anymore, so they train real hard. Now everybody does it. Thus you have a lot of people suddenly able to do these feats.