in many a place not having the tax to meet your liability is your problem. At the moment of the taxable transaction you did have the money to pay it. It's not the tax man's problem if it's subsequently been lost or bet elsewhere since or more likely the overall value has just gone down.
Hate to say it, but it's actually a good point and it makes sense. You could argue tho, that with crypto-to-crypto trade you don't earn "money", so you can't make provisions for the tax due, but still you could convert appropriate portion of your cryptos to fiat... This shit can be a minefield.