Why there is decreased urine output in respiratory disease ?

It is not a rule: I don't think that it is a rule. Urine out put can increase or decrease in respiratory disease.

Many possibilities: First it is not always the case that there is decreased urine output. If it is, there are a couple of possibilities. The main ones are 1) the patient is dehydrated, not drinking, fever etc , 2) if severely ill the patient may be "septic" with a low blood pressure and decreased blood flow through the kidney, and finally 3) direct injury to the kidney if severe lung infection and sepsis.