I think it's mainly because in 8-ball, it's mainly a short distance type of game (not to say you don't need to know how to shoot long shots when you need to) but if you play for the middle of the set of your balls in terms of placement on the table, usually you can play with some finesse/stun shots and you'll be set for the next shot. And like someone said, unlike in rotational based games (e.g. 10-ball/9-ball) you don't have to play position for a single ball. And it's not always that your balls are at the same half of the table so you need to use some pace/spins to get position up and down the table.
My friend is awesome at 8-ball but isn't that great in 9 or 10 ball because most of the times, I see him fall short because he hits them too soft.
This is very true. At least for me, it is. We all can make longer shots, but they are more difficult than shorter shots, and the more you have to shoot them, the more chances you have for a miss, or a poor safety.
My best buddy has been working with me for some time now to get better at 2 and 3 rail "leaves", for better position. He's been trying to teach me this for 8-ball, because that's what we play mostly. I can see how the multi-rail leaves would be of eve greater assistance for 9-ball.
When I'm referring to my 9-ball play, I'm referring to regular Texas Express (except in our club you have to call the 9, which results in plenty of "calling" in case you get a lucky combination, or carom - in our game you keep shooting if you make your OB, even if you are calling the 9) so I'm not worried about ball-count, I'm concerned with winning the game.