Why is it common for anime to lack surround sound? With all this push for HD video, I would think HQ audio (5.1 surround sound) would be obvious. But I am having trouble finding any TV anime in 5.1; One Piece from Japan is only stereo (movies are an exception), Crunchyroll does not have any 5.1 according to their support, most anime series listed on sale say on Amazon (or even Rightstuf) don't even seem to have a field for the sound type (stereo or surround). 

Why is this?

According to a reddit user:


  ISDB supports 5.1 but in practice most broadcasts are stereo, with 192k AAC audio.


[1]

Another question of mine would be why would the DVD releases not support 5.1?
Probably the only anime that I saw with 5.1 audio is Evangelion. However as you've pointed out, most(if not all) TV-aired anime in Japan are in stereo unless it got released into special BD/DVD.

This issue is also under budget constraints, etc. It's not budget-wise to apply one, just the same as what @Dmitri said about the average Japanese household.