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Using an established cryptocurrency doesn’t provide a good way for them to profit.

MobileCoin is pre-mined and the majority of coins are held by founders and the MobileCoin organization. They want to sell you coins, so partnering with Signal to force you to buy their coins to transact on the app gives them a revenue stream.

If they simply added Monero, they wouldn’t be able to sell you coins.

No near-instant confirmation, maybe. At first glance not a big problem for users, but can run you into a lot of problems

That's not a problem if you accept transactions in the mempool.

Only thing I can think of off the top of my head is optics (most of the installed userbase for signal has probably only ever heard of monero in a negative light).