I think lack of knowledge is the biggest.
Its hard to begin to eat well when you don't know reasoning behind it. All you know is "this is what so and so said worked for them so imma try that"
Once you realize how your body uses carbs proteins and fats, you begin to realize we consume a lot of stuff the body just cant process and or doesn't need.
Which leads me to the second reason: Portion control
This is big for Americans, I think reports have shown that we buy and consume the most food world wide. A lot of people tend to eat out of boredom, and or for taste, not necessity. And go WELL over the serving size when eating meals.
Third, people tend to want the best of both worlds, they wanna eat anything they can and still have killer abs for the summer. That only works for people for extremely high metabolisms, and even then, they aren't usually very muscular. Also, being skinny DOESNT mean your healthy. Seen so many people work out just to throw it all out the window from binge eating. At best you'll stay the same weight doing that. Working out doesn't give you a license to pig out. You can afford to cheat a little, but you'll never see significant progress with out a modest eating regimen (not diet).
Last of all, patience and will power. These are big, some people just don't have the discipline to say no. They don't see they benefits of it beyond losing a little weight. And that's fine, if your happy with eating what you want, and remaining the same size (or possibly gaining weight) cool. Just don't complain when you look in the mirror and you don't look the same as your high school self (girls do this so much its nauseating) or you just can't explain why you have random health ailments.
I honestly feel if you can conquer and achieve the weight/fitness phenomenon, you are better for it. The feeling of setting a goal for you self and meeting (and some times surpassing it) is like no other, and can make you feel like you can do anything.
-DMP-