cashu is built on LN so they'd have all the extra LN management cost and the cost of the headaches and time spent, for very little gain. while XMR is a breeze and just works. if there was on-chain ecash that would make more sense for their models, but then they could be ecash mints of both btc or xmr so at that point they still wouldn't be replacing Monero
Discussion
It also adds an additional rug risk to user and vendors; the market escrow itself(is multisig currently or theoretically possible with ecash?) + ecash mint. I doubt most vendors are going to manage their own LN node. Ecash is a big convenience upgrade for users though so might be cool to at least see the option.
I wonder if there are any mints that exist right now that are Tor-only
I'll admit, I'm not super well-versed in cashu, but isn't the point that you can just use whatever mint you want? I don't think DNMs would need to run their own infrastructure. Honestly, they probably shouldn't since you can pretty much guarantee everything happening in that mint is illegal activity.