If you look at the methodology behind this index, you'll see that it isn't really about walkability per se.
This index measures how far you have to go to get from your house to a store, a place to eat, or a job. If it's more than 0.25 miles, then it's not counted as a walkable trip.
Due to consolidation and the size of many of the residential developments that occurred here in the Seventies and Eighties, Jacksonville has way more than its share of trips over 0.25 miles, i.e. trips that are counted as unwalkable.
As far as this index goes, it doesn't matter that we don't have adequate sidewalks or bike lanes.
They are working on a better index, but these results are based on the old one.