From a quick look at the screenshots, and the other comments here: because it offers more than the basic spectrum view of Audacity?

There's a big difference between just showing a bucketed Fourier transform of the whole audible spectrum, and showing a high-resolution, log-scale, musically-informed visualization of the part that can be heard as musical notes.

More detailed dedicated visualisations, broader support for analysis plugins, and a focus on visualisation and analysis work rather than editing. (Audacity can use some of the same analysis plugins though.)

I don't understand why this question was downvoted.