Because in English, that second 'o' in such an awkward place would obviously become a schwa, and because the country's namesake is spelled with a 'u' in English, too?
"There’s a reason coke and heroin cost so much more on the street than at the farm gate: you’re not paying for the drugs; you’re compensating everyone along the distribution chain for the risks they assumed in getting them to you. Smugglers often negotiate, in actuarial detail, about who will be held liable in the event of lost inventory. After a bust, arrested traffickers have been known to demand a receipt from authorities, so that they can prove the loss was not because of their own negligence (which would mean they might have to pay for it) or their own thievery (which would mean they might have to die). Some Colombian cartels have actually offered insurance policies on narcotics, as a safeguard against loss or seizure."(source: http://www.nytimes.com/2012/06/17/magazine/how-a-mexican-dru...)