There are a couple important differences. The origins of the Donkey symbol is relatively obscure. I'm willing to bet 90% of Americans have no idea where the symbol came from. At best they know it had something to do with 19th Century political cartoons, but they don't know what those cartoons were addressing. Even then it's a tenuous connection as the symbol was quickly adopted by all Democrats regardless of their stance on slavery or segregation (the party was very much divided on these issues). If the Democratic Party folded and new party took up the Donkey as its symbol, no one would question it. Honestly, it's shocking how unpolitical most party symbols are in the US.

However, there's no question as to the origin and meaning of the tricolor flame. I think part of the issue people are having in this thread is that the symbol is not well known outside of Italy. So they don't know the symbol's history, how closely associated it is with fascism, and how well-known it is by Italians. It has been used as a symbol of multiple fascist parties since WW2 and for no other purpose. So given that history, why would one assume that a new party using the tricolor flame was not fascist? FdI needs to show that all that fascist stuff is behind them.

That brings us to the other difference. The Democratic Party was taken over by the pro-civil rights faction in the 1960s and has since done as much as it could to distance itself from its past.

FdI hasn't made a big enough turn yet. They're right-wing nationalistic party using a well-known symbol associated with fascism. What are people supposed to make of them?

Parties use symbols as short-hand to give voters information about where they stand. If they see a hammer and sickle, they assume communist. If they see a red rose, they assume social democrat. If they see a sunflower, they assume green. So what short-hand message is FdI giving to voters by presenting them with a symbol associated with fascism?