because it's more even, (i mean, 1/64th? that's just hard to picture) and you can use decimals more easily. With 32ths and 64ths you have to convert... unless it's half an inch or so.
the metric system is way better... and since americans don't want to use it, I guess One has to be satisfied with inches divided in tenths.
and since everyone that posted used decimals, I'm guessing they sell rulers with inches in tenths, since it would be less work to say what the ruler really says and not convert it.