[cmpb]

I'm from the area, so I feel I might be qualified to give you an answer. The short story is that some people here do still speak dialects of French [1]. The number of native speakers is rapidly declining (and will soon diminish completely), but many residents of Louisiana have grandparents or other family members who did grow up speaking it exclusively (or more commonly now, grew up speaking it with their exclusively-French-speaking parents/grandparents).

There are probably several reasons that it has held on for so long here, but predominantly it's because of multiple waves of influx of French-speakers (from when Louisiana was owned by the French, then from people of the Acadia region of Canada who were forced out of their region and migrated here in the mid-18th century) combined with persistent poverty resulting in poor education and low travel into and out of Louisiana (so not a lot of mixing with the rest of the US).

[1] https://en.wikipedia.org/wiki/Louisiana_French