Please note that you don't have to use lightning as gateway, you could also use stripe, PayPal etc

1. LN is a protocol and the "glue" between mints, thus the intra-mint transfers go as smooth as oil. The financial system does not have this kind of standards for small amounts, moreover minting and melting of tokens with LN can be done in seconds and does not need burocracy or other guarantees, which is good because if I mint a barer token to a user and I dunno the users at all, how can I reverse the financial payment that generated the minting of tokens in case of a fraud? With LN there is no such a problem: if the invoice is payed it means that the payer had the cryptographical right to sign that transaction, so we are fine with the immediate minting of ecash
2. LN can execute multi path payments, meaning that if I have 100 sats with mint A and 50 with mint B, I can send 130 sats on the LN to a mint (or user) C simply by requesting a MPP from mint A and B at the same time. This is not possible with the classical financial tools at all, so for fiat ecash every user would be limited to spend at maximum the biggest amount of ecash that he/she has with the same mint.
3. Because of the willingness to keep AML procedures in place (and to be able to extract taxes) ecash proposals with fiat all have one thing in common: privacy is not actually in place. For example, in GNU Taler the receiver does not have privacy and this """"feature"""" has been implemented to enable tax monitoring and AML. This means that the transaction cannot happen offline, leading to a HUGE limit in terms of technology. May be an implementation issue with GNU Taler and not a general problem, but I highly doubt that governments would allow a perfect privacy-oriented ecash system without even blinking an eye.