I think it's because most vehicles for trading ownership over companies or commodities like gold have a good amount of friction to them, making it impractical to deal with them as if they were money. This might include a minimal value of a sale (e.g. a stock is worth $20 and can't be divided further, although there are systems for owning a fraction of a stock) or trading fees. Bitcoin, on the other hand, is easily divisible into small fractions.

Not saying there might not be some way to make it work, but it probably involves a lot of unnecessary risk and high transaction costs.

Seems like it's just too much headache without any of the purported benefits that bitcoin has over traditional money.