Because you can find each mark just by folding the distance in half again and again.
Halve an inch and you get 1/2; halve that and you get 1/4, then 1/8, then 1/16. Every mark is reachable by repeated halving, so the spacing is easy to lay out and easy to eyeball. Tenths have no such midpoint shortcut, which is one reason carpentry rulers stuck with fractions.
