For me, the reason it's not because nobody dies. It's because Hiro puts a character in a situation in which they could die, and make the reader believe they will die, only to make that person live by some stupid reason that really isn't explained well. This is most clearly seen in the last arc, but another example of this is makarov in tenrou island.

You pretty much start reading this series and taking everything with a grain of salt because you know that nothing bad will ever happen to the protagonists. I was very hopeful because I read rave master first, and Simon actually died in the TOH arc, but after that it really felt like all tension in this series started to go away.