An asset is something that generates revenue, bitcoin does that.
Bitcoin does not generate revenue. Bitcoin goes up and down in market value, that is quite different.
A stock is an asset and is speculative for the most part people invest in stocks for capital gain,
While a stock is often a speculative investment vehicle, stocks are (at the core) contractual evidence that you own a definable piece of a business venture; which (presumedly) exists to turn a profit. It's the profit that makes the stock an asset, not it's speculative nature.
but that is fine if you want to call it a commodity instead of an asset
Nor is it a commodity, because it has no non-monetary utility. No one desires to aquire bitcoins for their own sake, but to spend or sell them at a later date. You can't really do anything else with them, at least not yet. (Colored coins might change that analysis later)
so long as you dont call it currency which it clearly is not.
Bitcoin is a currency, as I noted in the "bitcoin is not a currency" thread on several occasions.