#3 - It's an established series
Established series generally have the advantage over newcomers.
First, the company behind the former typically has some money at its disposal. Second, fans are more aware of its existence. Both factors have helped mold the GTA series into one of the most successful video game franchises of all time.
Almost every gamer is aware of GTA. Several controversies and praise have made it very much a part of a gamer's public conscious, so Rockstar Games often doesn't have to do much for players to be hyped for an upcoming game.
That Take-Two Interactive Software spends a lot on marketing only helps make Grand Theft Auto even more popular among gamers.
#2 - Players are free to do whatever they want
There is no obligation to do missions right away. If players wish to spend ten minutes looking around doing whatever they please, they are free to do so without the game stopping them.
Likewise, they can spend dozens of hours collecting everything and exploring the world before getting back to the storyline. GTA often rewards exploration, as players can find Collectibles (hidden packages, for example), weapons, and vehicles all over the GTA world.
Considering the general gameplay is so good, gamers often enjoy this level of freedom. They could even do specific missions in whatever order they please, and that's not even scratching the surface of how much freedom they have in GTA.
Some games often have a point of no return, where they might not ever go back to a level (or have to use a level select feature in other games). GTA doesn't have that; the cities of Vice City, San Andreas, and Liberty City don't vanish outright (aside from GTA San Andreas briefly having CJ be in a small part of Liberty City).
#1 - It allows gamers to live a different life
It should go without saying that most people shouldn't be involved in drug trafficking, numerous assassinations, and hundreds of bank heists.
Logistically speaking, most people would fail at any one task listed above, but trying them all is outright impossible. Even if they tried to live lives like that, they would likely die or get incarcerated for a long time.
Fortunately, gamers can play GTA to live out some of their wilder fantasies. There is no issue with being Wasted or Busted, as they always respawn. They can live as dangerously as they want to, have the fanciest homes, weapons, cars, boats, etc., without worrying about potential ramifications.
This is especially true for gamers who play multiple GTA titles, allowing them to experience more variety.
Note: This list is subjective and reflects only the writer's opinions.