I'm not really a finance guy, but just off the top of my head, explicit credit requires trust, which mostly defeats the purpose of the blockchain, where you're trying to transact without needing trust.
I also assume that explicit credit is more expensive administratively than the kind of liquidity provided by a clearinghouse. I guess technically the NYSE (or whoever the clearinghouse for the NYSE is) is extending traders credit in the sense that it must believe they are good for the trades they make without requiring them to post full collateral for every trade, so in a way, it is explicit, it's just not bilateral between each pair of traders.
Once you start to do transactions that involve transfer of anything off-blockchain (ie. any really useful transaction) you still have the same issues with trust and credit as without blockchain or whatever. What blockchain solves is somewhat fast settlement of monetary-ish transfers, which to me looks like a problem that does not really exist. (In my exerience SEPA transfers are mostly instantenuous and generally irreversible)
Why not just have explicit credit instead of implicit credit?