That's a myth. Satoshi compared it to gold multiple times. Imagine if gold was digital: You wouldn't be able to make it fit into a single category like "currency" or "investment". Or maybe you could, but you would have to leave behind this common modern assumption that a currency must have the exact same value every year, or even lose 2% every year.
Too many investors not enough users?
As someone who owns no bitcoins I find the whole thing fascinating to watch.