Pitchers are throwing 100+ MPH. They've got sharper curveballs, heavier sinkers, all of that kind of thing is putting much more stress on arms. Batters are hitting 50+ HRs, running faster, jumping higher, swinging harder. Overall the game is being played at maximum speeds that are testing the limits of the human body.
I think also the game has become more and more filled with "specialists" that are really, really good at one thing but aren't as well rounded. Just a guess, I'm sure there are many reasons.