He probably sold more records than anyone else beside the Beatles.
That applies only to (North) America. The rest of the world don't even have a clue who Garth Brook is.
Not true; he sold out multiple nights in Dublin in a matter of hours. The shows were later cancelled because of zoning board issues, but still. If memory serves, he's big in Australia as well.
Any regional "lack" of popularity is as much a function of the genre - American country music - than it is him. I have ZERO doubt that if he wanted to, he could be a massive rock star, metal star, etc.
Having said that, SOME of his sales success is marketing. He's played the game magnificently. His first box set was his first six studio records repackaged with one bonus track each; each sale of that box set counted as SIX records, AND counted as another for each of the individual records. His second box set was a similar package, starting with his seventh album ("Sevens") and the double live set. Then he put a box set with I think four CDs of cover tunes - rock, country, etc. - modeled after his Vegas residency, and repackaged in his greatest hits CD. So every sale of that box counted as 7 disks (there was a DVD too) AND his greatest hits rang up numbers as well.
Notwithstanding that gamesmanship, I love the guy. He's more or less sincere, he's very talented (I've seen him live twice and both times were killer), and he makes music that people like. Not sure there's more we can ask from our artists.