Well it depends on what you find as a satisfying answer. I'd say that it decays into such, because "it can" . It's allowed to (due to the symmetries) interact/couple with those particles and so it does.
+ the ( W-> e nu ) is conserving all the quantities we think that are conserved.
P.S. - the title of the thread is misleading, you should try to write a title that represents clearly your question.