Decentralized exchanges have failed because everyone needs to be where the liquidity is. Much more so for payment systems. When someone cant route their payment because of insufficient liquidity, both the merchant and the customer lose.
Decentralized exchanges are still in a raw state. They are not newbie friendly, just like the Lightning Network. I wouldn't call them a failure because there is still a lot of work which needs to be done. We still lack user-friendly Lightning Network wallets which could provide channel backup and easy full-node setup process. There is a risk that the whole network becomes more centralized in the future because of people opening their channels to the biggest nodes or depending on a third party. Despite all this facts, the number of nodes and users is constantly growing and you must be asking yourself, why is that?
Lightning Network is still in its early state, why not compare it to the early days of Bitcoin when it was fairly easy to take over the network with a 51% attack? Give it some more time. Nobody expects to see every Bitcoin user using the Lightning Network this year.