Partly because it was poorly implemented. The clock didn't stop and restart when the ball went out, only when the ref signaled that the clock should be stopped, usually due to injury or something. With the technology we have now, it could be done better, but I don't think many soccer fans think this is a problem that needs to be solved. I think they solved the problem years ago when they started announcing how much stoppage time there would be. Back in the day, you literally had no idea. Now they tell you it's 4 minutes, and it's usually accurate. MLS basically tried to Americanize soccer to make it more appealing to non-fans, but non-fans didn't really ever get into it.
Ultimately though, an American style countdown clock in soccer doesn't really make much sense, because there's generally not a scenario for a buzzer beater. In the NFL, you could be anywhere on the field with 1 second on the clock and be able to score. Similarly with basketball, the court is small, and the clock doesn't start until someone catches the ball in bounds, so you can have a buzzer beater. That's just not possible in soccer. A countdown clock actually had the opposite effect, because when the clock ran out the game was over. In a normal soccer match, the ref generally gives an attacking team an opportunity to finish their attack, and only calls half/full time after the chance takes place. So if there's 4 minutes of stoppage time, and a team crosses midfield with the ball and is heading toward the goal at 4 minutes, they are given a chance to score.
Soccer fans generally didn't like it, and nowhere else in the world does it like that, so they switched after a few seasons. They also had hockey style penalty shootouts for a tie, which was kind of cool, but that also didn't last very long.
