A lot of the most popular iPhone apps haven't made the jump to the Apple Watch
The Apple Watch has almost certainly benefited from the huge iPhone install base, but a lot of developers of popular iPhone apps haven't brought their apps over to the watch yet.
This chart from Statista, based on data from AppAnnie, shows how many of the top 10 most downloaded iPhone apps in 2015 in each category are also available for the watch. Notably, only 5 out of the top 10 Health & Fitness apps have made the jump - kind of a surprise, given that the watch is being marketed as a fitness-tracking device.