Multiple reasons why MOB over monero but I will state only the most practical one, using the stellar consensus a tx takes 3-5sec. With monero that's more like 1-2mins if Im not mistaken which is a big no-no for the usecase of instant mobile private payments. Fees are also super low w/ mob (0.01)
XMR transaction can show up instantly once they are in the mempool, a user does not have to wait for a confirmation to see a transaction. Obviously there are uses where you want to wait for confirmations, but for the use case of sending money to friends it isn't necessary.
Using 0-conf is restricting usage to "friends only" which is a tiny subset of who you may want to send money too so I think you've already answered this.
Also moxie replied in his twitter about why not monero stating they wanted a "non custodial" integration, not sure what he means but you can ask him.
TLDR: UX