That isn't small for a production run in one year. Some models only run 5 to 10K a year. Hell the Ford Taurus, a car heavily bought for fleet use only sells between 50-70k units a year.
It's a small number. Ford f-seies pickups sold over 60,000 units in Feb. 2016 alone. And there is a helluva lot more competition in the pickup and midsize vehicle categories.
In reality, since the Excursion is no longer produced the Suburban doesn't have a competing vehicle model.