Could be due to the fact that a lot of people don't have a weekly need of a maps application. I'm a heavy phone user and google apps user but only need the maps app maybe 2-3 times per month at the very most.
It's one step forward (from Apple Maps), about 800 steps backwards from the original Google Maps app. You can't even save bookmarks to locations without logging in to the Google/NSA overlord collective. Not to mention the horrible UI that makes Dwarf Fortress look reasonable by comparison.
Yeah, the title should be, "Although Google Maps remains a popular iOS app, Apple Maps commands a wide lead over Google Maps in overall usage statistics". Apple Maps still gets me lost on a regular basis in major cities.