Mental toughness. You throw a young forward into a game, and he makes a mistake, and most fans don't know. A young defenseman makes a mistake, and some people can see it. When you throw a young goalie into a game, every mistake he makes is magnified. That can crush a kid.
Plus, technique now compared to when Barrasso played is like comparing stick technology from 1980 to now. No comparison. Barrasso couldn't play in today's game without some prep time prior to stepping into the NHL. Goaltending techniques are difficult to master, so you want to start a kid at a level where he can learn and master the position against shooters who aren't going to rip him up. That's why goalies regularly start in the ECHL.