Because the current US govt definition of "broadband" is a minimum of 25mbps down, and most DSL can't achieve those speeds.
According to footnote 11, the FCC data set uses 10/1 as the definition of "broadband"
That's a recent change, broadband used to be defined as 25mbps, and that definition was recently lowered to 10mbps.
But for what it's worth, a lot of US DSL still wouldn't meet broadband definition, even using your lower speed definition. AT&T DSL (the non-FTTN version) in Michigan maxes out around 6mbps down for most people.
I have two DSL lines, one 1 Mbps, the other 2 Mbps thanks to a patented range extender is no longer manufactured because the patent holder got acquired.