There's a shitcoin that tried this. Decred forked BTC with an incentive to participate in collaborative transactions.

As a result, the majority of transactions are part of a large-enough anonymity set that most of the Decred chain has gone dark.

It's as if JoinMarket was actually easy to use.

A quote from Decred developer:

"To do a coinjoin you need some type of coordinating server for multiple parties to create the shared coinjoin transaction. Prior to joining Decred, I wrote code for the joinmarket project which was the leading bitcoin coinjoin creation software. It tried to increase mix volume to build a large enough anonymity set by using a market to incentivize others with profit to offer their coins for joining. This model worked but never really picked up enough steam to build a large enough anonymity set with many participants to make the privacy resistant to chain analysis. Less then 1% of coins were participating. This became known as the problem of coincidence... needing people to be sending transactions at the same time for the same amount. While writing code for Decred's ticket buying system in 2016, I came to see the ticket purchasing architecture was the key piece that was missing. Decred's ticket buying system uses transactions of the same denomination that occur frequently at the same time, and with a large percentage (50%) of the total coins existing participating. It was from this insight that the initial prototypes were created."

I'm holding out for JAM to make JM sexy. Perhaps cross-input signature aggregation makes coinjoin cheaper?