It's because as a carry your play style is nearly the same every game. You're going to run around and hit creeps and get as much farm as you can while helping your team when beneficial. In most scenarios your gameplay boils down to "Can I get this farm without dying?" which is relatively the easiest question to answer in dota.

2-5 have a lot more guess work to do with figuring where their correct movements should be on the map - all the while having a much smaller resource pool to work with. You have to know the tempo of the game, your win conditions, what weak points to exploit on the enemy, how the enemy is moving, etc. All of these complexities determine where you're going to move your hero on the map. Meanwhile the carry is just running from creep to creep as long as he knows he won't die. (This isn't 100% how every carry game goes - sometimes you have to be more proactive, but this is high risk and often times a result of a bad or cocky draft).

However, messing up as a carry is extremely punishing, and often times loses games, which is why you don't put any idiot who can hit creeps on pos 1.