Liquidity, speed, and cost but it's somewhat subjective and depends on what matters most to you. Personally, I'd rather be able to spend directly, without the trials and tribulations of offloading into something more widely accepted.

Nothing prevents you from building one that supports Monero, Nostr can do all things if you just wish hard enough 💫. Adding swap functionality on top is another feature thats up to clients if they want to build that out. But I personally think the cashu model is the way to go adoption wise if you looking to keep it on a bitcoin standard. Or ark stuff but that’s hasn’t been around for that long yet.