It's the bullpens. I've done a ton of juice leagues where teams from every era compete and teams with modern day bullpens tend to be above average. Back in the old days, relief pitchers were guys who weren't good enough to be starters. Nowadays, we have specialists like closers, set-up men, and left handed specialists with ERAs under 2 and whips under 1. That was a rarity back in the day.
Also, mid-century pitchers tended to both start and relieve. So, you'll get a lot of guys who averaged 3-5 IP/G. In RL, those guys could throw complete games. In the sim, you have to set their pitch count to 50-70, so they'd never finish a game. And unless you tinkered with your pitching staff on a daily basis, they would pitch less than the number of innings they did in RL.