What is the criterion for deciding weather a bus route is worthy of single or double decker buses?

I am mostly questioning the 165 service from Rainham - Romford, this service was nearly always double decker up to around late 2001, now you have to be so lucky to get a double decker on the 165.

I do realise that there are sometimes when a single decker is fine on the 165, but if your ever around Elm Park when the schools are "chuckin' out" the 165 single deckers are packed and have to miss stops