Don't just make this about the French left.
Abstainers are surely those on the left who view Macron as a neoliberal stooge.
We know this based on how first-round parties behaved (Far Right non-FN endorsed FN, everyone else endorsed EM, except Mélanchon who endorsed Being Really Angry At The Second Round!!!!) and also based on the plain reading of those who abstained describing why.
It's fine that there are those on the left who disagree with me and sincerely believe that Macron is a far right conservative liberal neoliberal guy who is going to ruin France, but I feel very comfortable dismissing them as misreading the situation entirely, engaging in hysterical theatrics, and lacking context to other elections or to other countries or both.
If we are to talk about differentially high abstention in this election (as opposed to structural abstention across all elections, or as opposed to a general trend towards abstention in the OECD countries), it is surely a phenomenon of the left.