...I understand why Haruka is there, but... why is Reimu there?

most likely the three favorites of the artist or something.

The Big Top Three of pixiv and Niconico Douga. It isn't very hard to understand.

Sorry if I missed the boat, but is idolmaster really as popular as touhou and vocaloid?

as far as Niconico Douga goes, just about. In any given video, you'll see all three of them.

Ranranrun said:
The Big Top Three of pixiv and Niconico Douga. It isn't very hard to understand.

ah i see...sorry to have missed that