Le Pen didn’t exactly have much of a economic plan...her agenda was with asylum and immigration...fixing that. If the National Front had spent two or three years talking about the economy and put effort into a real program....they would have gotten more votes.
Same problem with Germany’s AfD Party....lot of hype over asylum and immigration...virtually no talk over the economy other than wanting some taxation cuts.
There is also that remaining stigma having to do with Jean-Marie le Pen. His daughter is doing nothing to erase it.
I saw an hour-long interview with LePen last year or perhaps the year before...she's a stone-cold socialist. Lots of talk about nationalization of key industries and so on. Maybe the French just didn't want that crap.