A large part of the appeal was just how real it felt. 90% of scientists or engineers in movies don't talk like real engineers do. I thought that being in the situation where you understood bits and pieces, but not everything as a whole put you in the same situation as the characters were in - of having discovered the ability to time travel, but not being fully aware of all of the implications of this Why do you think that it was muddled rather than confusing? It takes a while to understand, but when you do understand it, it all makes sense
Because it's not a complex movie. It's the Tragedy of Dr. Faustus by Marlowe. Yet people with no real understanding of narrative structure presume that because things are so garbled and take so many attempts to understand, it must therefore be brilliant rather than poorly executed.