Money and tradition, but mostly money.
It's not a league like the NFL or NBA. FIFA oversees and implements the same rules for soccer at all levels, in all countries. It's the same rules for everyone, everywhere. To do that, it couldn't just be implemented at the WC level or in the EPL or Serie A, it would have to be implemented through every division and the entire pyramid of every country going down to the English 6th division and 7 yr olds playing in their local AYSO league on the weekends. The NBA and NCAA can't even agree on a shot clock, 3pt line or how long a half/quarter is. It's a fair bit more to ask for every soccer league in existence to change.
To an extent, an extra 30 secs to a min of extra time is left to the discretion of the ref, like the ball/strike zone in baseball is up to the discretion of the ref. It's confusing to some, just as one ump calling a ball where another calls a strike, but that's part of the tradition the game was built on.