Well, it's because originally old French had two competing number systems:
One that followed the general romance model of having specific words for units of tens, so dix, vingt, trente, quarante, cinquante, soixante, septante, octante, nonante…
And one which was a simpler composite system in base 20, possibly influenced by celtic languages, so dix, vingt, vingt dix, deux vingt, deux vingt dix, trois vingt, trois vingt dix, quatre vingt, quatre vingt dix (it's simpler in the sense that it needs less words: once you've learned to count up to 20, you already know how to count up to 99, without having to learn additional words).
Swiss and Belgian French eventually completely settled on the first system, but the French of France settled on a hybrid system.
It's worth noting that people who count like this don't literally hear "quatre-vingt-dix" as "four twenty ten" — they treat the phrase as a unit that happens to mean "ninety" — in that sense either way to say it is no more or less arbitrary.