Historically, rhenium was alloyed to the tungsten in the filament of valve tubes to increase electron emission. What is the major reason this isn't used in x-ray tube filaments?

usually x-ray tube filaments are devices that limit the current flow i. e. current cannot easily pass through them or we can say that they are current limited devices. So to allow fine electron transmission control by filament heating current, rhenium must not be used in x-ray tube filaments.