Why does the hole purchase have to be funded with BTC

Why does the BTC transaction value have to be funded with BTC plus fees, when the transaction is being paid with Zelle? Why won’t the security deposit do.

If you are buying BTC with Zelle, you still need to put a security deposit and fees in BTC, because there is no way to lock fiat in a multisig like you can do with bitcoins. Security of Bisq depends on these 2-of-3 multisigs.