Not for self-preservation, but because they were expected to pitch 9 innings. Nowadays, it's throw as hard as you can, and maybe get to 100 pitches if you are considered rubberarmed.
I'm not saying they were doing it consciously, but there is a different attitude when you are trying to get a ground ball, versus strike outs. Go back to Ryan's heyday, and hitting the 90s meant you were a flamethrower. Lots of major leaguers couldn't break 90. Today, even some high schoolers are. A lot of MLB pitchers today have changeups in the 90s.
Verlander talks in that documentary about the progression from the 90's being the flamethrower to guys pitching 100 now being the flamethrowers. Ryan had a few things to say about it as did Tony Gwynn. I'm going to watch it again later today.
The moments they reflect back on Koufax, Gibson, Walter Johnson with guys like Hank Aaron talking about what it was like facing them are special moments. The interview with Schmidt talking about Ryan throwing high and inside to intimidate and take back the plate made me laugh. Oh, and the segment on Goose Gossage was great. I forgot about him.
They also compared the difference to how pitch speed is measured today to how it was then and had a physicist from Carnegie Mellon speculate on just how fast a Nolan Ryan pitch may actually have been if measured by today's standards.
-Mike