why are jazz musicians so passionate about harmony and voice leading?

It's difficult to know what other people are thinking without talking to them directly. With this level of information I would make only two points:

1) It doesn't count as "passionate about harmony and voice leading" unless they understand Westergaardian theory well enough to contrast the two. Otherwise it just amounts to "passionate about music theory of some kind".

2) It doesn't have anything to do with jazz. If they're right that harmony is the superior theory for jazz, then it's the superior theory of music in general. Given the kind of theory we're looking for (cf. Chapter 1 of ITT), different musical traditions should not have different theories. (Analogy: if you find that the laws of physics are different on different planets, you have the wrong idea about what "laws of physics" means.)