You said 1:40 so I'm going to go with the Youtube version, as the Soundcloud one is a few seconds off of what I think you might be talking about.
That song in particular has a number of inspirations. Like many things, djent appeals to me when used correctly. Djent in a specific mathematical rhythm in 19/8 is very
appealing to me. It is just as complex to cram a series of close frequency notes into a single lower register [ http://www.youtube.com/watch?v=EkLZVs6D5os
] as it is to bring several common chord progressions together with multiple instruments [ http://www.youtube.com/watch?v=kWTu0fBIZqk
], as they have very similar structures in the grand scheme of things. They are simply separate kinds of complexity, entirely dependent on how you individually contextualize complexity.
So in short, I did it because it sounds good to me, but the long answer is up there (And down below). That whole lower stretch is quite complex, despite its repetitive appearance, and to me, sounds fantastic as a single instrument. An electric bass would have also been nice there I guess, but the piano is much more suited for my purposes. The song tells a story through a method of going through several ideas behind Touhou soundtracks, but not necessarily their direct chord progressions or melodies. That particular part was supposed to represent a bit of the darker side of Touhou, mainly Flandre, but not directly her theme but rather the general feeling around her character, her attitudes, and her location below the Scarlet Devil Mansion.
I leave it unresolved before 1:40, and when other chords come in right before 1:40 they are purposefully dissonant. This is the build to the semi-finale, you could say.
Melody again at 1:45, still not quite resolved, the complexity adding to the tension. While the lower register at this section is still a harmony in the traditional sense, it's complex enough to be its own melody in context, so the two melodies hammer out and compete with one another, each individual note from both forming an interesting and purposeful pair.
1:59 ish chord is the full resolution in terms of tonality, and the added third track allows me to build what most people would call full chords in every beat. However, it starts to move into other areas, transitioning into the big booming chords of the upcoming phrases, then referencing some Flying Bucket Adventure, then UN Owen Was Her, etc. But all of that build would have not had as strong an impact on separate instruments because it would not have been nearly as obvious. Plus, as a general rule of thumb, Touhou-related songs should sound good in almost any timbre, but sound great on the piano.
That's the general reasoning, anyways, I mean I only spent like 5 hours writing it over 2 days. Give me a week and some inspiration and I'll give you a 16 minute orchestral piece. This one was mostly mentally written out before I started.