Hey mate,
It's because I don't see that this attribute has crucial important for any position.
I'd say If a player has it then good if he has not then it isn't tragic at all.
Here's how "Balance" attribute works in the ME:
1) When a player with the ball then "Balance" helps dodge tackles.
2) When a player without the ball then "Balance" help to stay on feet after a player produces a tackle or turns.
For example, if a winger is fast enough and has good dribbling and high flair then with such good attributes he just won’t allow opposition defenders to tackle him so in this case "Balance" attribute is useless for him.
Of course, even the best winger still will tackled time to time and when it happens "Balance" will be useful but it would be also fair to say that sometimes wingers have air challenges and when it happens "Jumping Reach" attribute would be useful for them but I won't add "Jumping Reach" attribute as recommended attribute for wingers.