Decentralized exchanges do not have enough volume, but this is due to popularity. When decentralized exchanges become more popular, then the problem with small volumes will be solved by itself.
See volumes are decreased due to because of ETH price, once the price of ETH was around 1000$ and the volumes of the DEX platform are very high and now the value of ETH is around 140$ which makes low volumes. I hope the Decentralised platform will more in the coming days because every country coins are launching their own blockchain platform as a smart contract platform.