Colour , honour , humour ... U
It's because Americans don't sufficiently value astrology. If they did they'd know all the people born in April (Aries, the Ram) would spend their formative years looking for U's (ewes). So, in order to discredit astrology, they drop them out.
In Australia we not only use the British spelling and include U where it belongs, but we even give it more weight when using the vowels in second person plural form. That is, A, E, I, O and Youse.
Wasn't it GBS who said that England and America were two countries divided by a common language?