Yeah, probably what Dani said. Basically all the Beginner's Box encounters are designed to "lead" your players to correct/optimal choice, or to teach them mechanics one at a time. So this very difficult check on the iron door teaches them that they need to use their more niche skills, instead of just brute forcing their way through every option.
I am learning in my own campaign that if you let players choose between Athletics or any other skill, the fighter pushes the other person out of the way and chooses Athletics 100% of the time. Crafting check to make a support/ramp to move a large stone, or just brute force it? Brute force all day. And since Athletics has use in combat and common skill checks like climbing up/down walls, it's important to let other people like the rogue/wizard use their skills too.