Two Dog coins can be exchanged for one ONION coin. The price of ONION is so low, but the transaction volume is still very low. No one buys it at all. Why?

That is a cute but silly comparison. DOGE is near the end of a week long artificial pump (see discord /wallstreetbets pump and dump group. I do not know if they are associated with /r/wallstreetbets).
1 DOGE will soon be back to 1 DOGE (< $0.01). DeepOnion development continues with a dedicated team and will increase in value naturally with no need for a "pump".
Your answer is even more ridiculous and stupid. The price of ONION has been falling for three years. I did not see the value of this coin you said. Which valuable coin do you see has been falling? The value you said, the market does not Accept, and you are still narcissizing yourself.