Polling data. In head-to-head matchups Melenchon does worst against Le Pen (and it's not close, two-thirds would vote for Le Pen over Melenchon).

A candidate who needs to win against a far-right candidate needs to not alienate either centrists or the left. That means a strong core message focused on issues rather than ideology or radicalism. Biden/Harris didn't have a strong core message and alienated voters on the left, Melenchon is far too much of an idealogue and radical so he's alienated the centrists. A lot of centrist voters prefer Le Pen to Melenchon because Melenchon turned them away, not because Le Pen was so attractive.