Sales were steadily increasing from a low point in 2002, but they spiked from about 80k sales in 2006 to almost 120k sales in 2007. Those numbers are based on calendar year though, so maybe sales were driven by discounted TJs in addition to the JKU?
https://en.wikipedia.org/wiki/Jeep_Wrangler#Sales
Lots of reasons already mentioned for the TJ to be on it's way out, but no doubt the increasing popularity of larger SUVs paved the way for the JKU to dominate the market. I see so few 2 door JKs and JLs on the road that they look out of place.