OOP won out over imperative because it fits very well to (desktop) GUI programming - UI controls maps elegantly to objects with state. OO turned out to be broadly useful, but GUI programming was definitely the driver.

I don't think anybody had compared maintainability between similar projects in OO and functional style. Functional was not really a contender in this space.

Almost everybody who adopted OO migrated from imperative. The popular OO languages were imperative languages adapted to OOP - Visual Basic, C++, Delphi etc.