Because the volume of transaction for these currencies is too low for the market cap to mean anything. Imagine if there are, say, 1 million fancycoins in existence, all owned by me. I manage to convince you to buy one fancy coin for $1. Does it means that I now have $999,999 left in fancycoins "market cap"? In theory maybe, in practice I'll almost certainly never manage to liquidate them at this price.

For these small coins like DeepOnion you'll crash the market completely if you try to sell even a moderate amount of coins. There's simply not enough demand to move a big amount of coins at this price.

Because the market cap of the tiny "currencies" is total bullshit and is based on two guys trading with each other, it's like me buying 0.0001% of your company for $1 and then you claiming it's worth $1M.

Bitcoin Gold is not seriously used by anyone.

The larger currencies are slightly more reasonable but even then the valuation is unhinged because, for example, a lot of Bitcoin has probably been permanently lost.

This is enough to do some double spending (= you won't get much here!)... if no one is noticing (if hey notice they can increase the cost / decrease the reward).

I guess some people still think DeepOnion have potential in the future...