There has been very little hype around the match so slow ticket sales isn't much of a surprise. The U.S. plays Honduras regularly so the opponent isn't bringing much excitement, plus the support in South Florida hasn't always been sterling. Toss in that it is a Tuesday friendly and it comes on the heels of the Landon Donovan farewell match and it's easy to see why people aren't dying to get tickets for the match.
It will be interesting to see how ticket sales go in the last 12 days before the match. They usually pick up as the event gets closer, and that's doubly true in Florida, where day-of-game sales have always been strong. But this has a chance to be one of the worst attended matches in a long time. It bears watching.