I would argue that You can't understand why it went viral by just looking at the qualities of the song itself.
Going viral is a feature of the social sharing, which requires the study of humans not merely of the music video.
So points made about kpop being unusual, and foreign for most audiences are just about the only ones which start to answer this question.
Other things make an audience receptive - pointing out the beat/style/catchy-ness mere highlights some necessary qualities of the song - other songs with these qualities may not go viral, but other songs without can not go viral.
However pointing these out in the context of what other people already k ow - accessible but alien - or contrasted with other musical genres which are already 'in the wild', will give some extra information.
However what is already out there is changed by this one video; it is comparable to an avalanche, one it happens the shape of the mountain side has been altered; another avalanche will happen, and it may be sooner (because the new shape is unstable) or later, Also it ma be bigger or smaller (again depending on the new landscape)
In term of the musical landscape, overplaying some music does make it go stale, and if some people keep hearing it (because it is everywhere) this will disincline them to share/listen to another similar piece, just like an avalanche can create a more stable system - this person is less likely to share that new piece.
So it was well timed, like sowing seeds on freshly tilled soil. The fertile minds were ready to pick up this meme and spread it.
Personally i think it is crap, can't stand the cheese.
Notice, 50 shades of grey and the london riots (2011) followed a similar pattern. The ground was ready for a big event.
This kind of behaviour is called self-organised criticality.
The other example doesn't seem the same, the 2012 London Olympics spread excitememt/happiness like a virus, but it was designed, constructed to be the event it became... We seem more impressed when things happen by accident; it would far more impressive to orchestrate such an accident!