Because FUT is so fun and so annoying at the same time also theres no other football games, while theres PES which no one likes

FIFA is a really popular game simply because football is a popular sport, the main game mode everyone plays is Ultimate Team where you can build your own team from players around the world. You can open packs with coins or FIFA points to build your club.

FIFA is so good for many reasons.
Its addictive, great to play with friends and always something to do on it.

OFFLINE GAME MODES.
Kick off- pretty basic, you pick a team you want to play and a team you want to verse. Its just a regular match.

Career Mode- Can be done as a player. You can create a player or play as a actual player (Messi, Ronaldo etc...)
It isn't as advanced in the sense that the NBA2k career mode is. You just play games pretty much as your guy and move to clubs if you wish and what not.

You can also play as a manager (its pretty fun) You control your squad, buy players, sell and loan players. Choose your squad, player contracts. You can play the games or sim them as well.

SKILL GAMES- Just little mini games pretty much, lick kicking the ball in buckets and stuff.

ONLINE GAME MODES-

Seasons- pick a team you want to use, and play against other people online. Progress through 10 divisions to reach division 1 by winning games.

Friendly Seasons- Play against your mate, you pick a team, he picks a team and play eachother.

Pro Clubs- In a way like MyPark in NBA2k where you control your own guy. You make a player and use him online in either a match where iys you and all these other randoms. Or if you got mates with the game, you can start up a club and get your club from division 10 to division 1. Its challenging, me and my mates are in div 4.

Ultimate Team- Probably the most popular one. Build your team up from pretty much nothing into something nice.
You can do online or offline seasons, going from division 10 to div 1 and earn coins from playing games, getting promoted, relegated or winning titles.
You can do tournaments and win coins and packs.
You can play your mates.

You can buy players of the auction house and add them to your squad or get players and other items by opening packs. (If you like ultimate team, then I recommend spending $20 on packs to start up.)
You can also buy coins (Which I have done and many other people) buy them off ebay or coin sites, but EA frown upon it and you can get banned if you get caught twice.

here is my team that I have so you got an idea
[ Register or Signin to view external links. ]

I would say get this game if you like football/soccer, or if your mates have it.