The coordinate question is usually served by почему́(počemú).
Unlike English because, Russian потому что can never govern an introductory clause (e.g. English "Because I was early, I had to wait"). For that, поско́льку(poskólʹku), та́к как(ták kak) and other conjunctions must be used. This separation of usage is not general to Russian conjunctions and is rather a peculiarity of a few including this one.