Why can the compound CH3CH2Br be called either bromoethane or ethyl bromide?

Answer 1

That depends on which system of nomenclature is being used.
Most standard today is IUPAC, which is most likely to be used in current scientific publications. Older names hang on for common compounds in industrial or pharmacuetical use because of branding issues, marketing, cataloguing, etc.
Bromoethane is the IUPAC name.