Why not the piano as a reference for bass

Personally, I think it is because piano live is amazing but it is difficult to reproduce well, so most systems would fall short.  I have learned that over the years about horns.  I tend to agree with you though.  A good sample of test songs should include horns and piano to test a system’s ability to enjoyably reproduce those instruments.  I don’t listen much to piano, just not my thing, but have really started to enjoy horns - feels odd to even type that because when I first started my system made horn sound terrible.  

Piano goes low but wave length impact too short 

A pipe organ much better there 

Depending on music type 

The shortness of the piano tone is the point. Stopping the overhang in the bass is a major problem for most speakers and the piano is a great test for this.

@freediver 

The piano is a stringed instrument. It is one of the best to evaluate gear. If our gear can't make a piano sound real, we are missing something. I don't know the lowest note on a piano (in hz) but it is quite low. Cello lowest notes are 30 hz, I believe, also a nice indicator of how your system sounds. Pipe organ is probably best for the very lowest notes or some synthesized music as there is pretty much no limit to what digital is able to do. Lowest pipe organ notes are 8 hz from 64 foot pipes but more recordings will include 16 hz from 32 foot pipes. It's the lowest I've heard on a recording (D2D vinyl).. I believe the piano is one of the toughest instruments to reproduce through a stereo system and many do use it as a reference, including myself. Not necessarily for the bass but the bass is indeed quite low. 

I think that the "attack" has a lot to do with it.  Whereas a bass is "plucked" by hand or played with a pick, a piano is struck with a felt-covered hammer and this presents a completely different "leading edge."