To me, the tempo of the music is the primary factor in what makes a piece of music relaxing. In the video above, the bpm hovers around 55-60 which makes it ideal for synchonisation to your brainwaves/ resting heart rate. (source)
While listening, your heart rate gradually comes to match that beat. It is important that the song is eight minutes long because it takes about five minutes for this process, known as entrainment, to occur. The fall in heart rate also leads to a fall in blood pressure.
Basically your video goes on for almost an hour, which is plenty of time for your body to slow down to match the music.
There is no repeating melody, which allows your brain to completely switch off because you are no longer trying to predict what is coming next.
Instead, there are random chimes, which helps to induce a deeper sense of relaxation. The final element is the low, whooshing sounds and hums that are like buddhist chants. High tones stimulate but these low tones put you in a trance-like state.
The video above is very atonal and there's a constant low organ-like tone that probably contributes to relaxing the listener.
There's a very questionable "academy" (homeopathy, basically) called British Academy of Sound Therapy which you can read more about. Google doesn't reveal too many of their techniques, but apparently there are lots of people dedicated to figuring out relaxing sounds.