If Kristin really is publicized more, it's probably because she's known more outside of the theater community than Idina is. If you were to say something about Kristin Chenoweth to someone who doesn't care much about Broadway, there is a chance they'd know who she is. If you mentioned Idina Menzel to the same person, you'd get "Idina.... who?"
Kristin is probably publicized more to draw different types of people to the show, if that is in fact the case.
A work of art is an invitation to love.