[edit]
Wikipedia suggests it might be from Wāpuro rōmaji, where "u" is always used to spell the kana "う"
reply
Many, but not all long vowels in japanese follow these:
ああ a i -> as in おかあさん, mother
いい i i -> as in ちいさい, small
うう u u -> as in すう, to smoke
えい e i -> as in せんせい, a teacher
おう o u -> as in こうえん, a park
Yes, exceptions to this exist (like おお) and some are actually dipthongs and not actually long-vowels, but easier to think of them like that.
[edit]
Wikipedia suggests it might be from Wāpuro rōmaji, where "u" is always used to spell the kana "う"