What makes the sicilian so good?

What is it that makes the sicilian so good? What advantages and strategies does it give players that makes so many masters play it?

In brief, an asymmetrical gameplay with good counterchances. 

It's a very aggressive Defense for Black. If White isn't careful they can easily be blown off the board.

Flexibility , structure is not fixed -(black can go for a6-b5, d5/e5 break even sometime f7-f5 break ) so there are many posibilities for black, and unless structure is not defined, for white is not so easy to create straigh plan like in other openings.

good counterplay on Queen side, c-file...

Perfect defense for the player with Black who wants to play for a win. The defense is rich in tactics. When players like Fischer and Kasparov make it their favorites you know it's good.

Flexible center, clear plans, dynamic positions.

Sierewan has a good video on the pin variation that answers this.  It’s one of his videos for the St Louis Chess Club channel.  The most basic part is giving up a flank pawn for a center pawn is strong, as controlling the center is basic.

The Sicilian tends to offer sharper and more unbalanced play. The majority of Sicilians are concrete in nature, and although ideas play a large role, theoretical knowledge is just as important.