Why is door pronounced as in 'o' not as in 'u' ?
Because door is one of those words that have two O's because people thought that would make the O sound longer. Meaning longer in duration. In Old English the word was spelled dor with a long O. There was no character to write that kind of long O, so they figured the double O would make people hold the duration longer. Maybe it worked for a time, but obviously it failed somewhere over the past 1,000 years.
It comes from middle english dore. The pronunciation of the word didn't change so much, but as spelling rules changed, it's spelling naturally had to change.
There are other words that are formed in a similar way from middle english spelling; more -> moor, povre -> poor.
Short Answer: There are two sounds for "oo". One is more like "Ô" and the other is the longer sound.