Because there are far more people and kids who like models, but don't want to build, don't think Diecast kits were a success either.
And creative kids use Lego and the like, to express themselves, maybe due to the fact that building a model kit, is a one way street, once build it just sits there and deteriorate over time (mom cleaning)
Lego can be disassembled and the parts can be used to build something else, dunno just guessing