1) Brevity. With four players and just the basic set, everyone gets fifteen to sixteen turns, depending on their seating. That makes for a pretty quick game, which is handy for a tournament where time is a definite limiter.
2) Small Foot Print. With only 72 tiles, including the starting tile, you have a fairly small board. I would even go so far as to say claustrophobic. Again, handy for tournament purposes.
3) Luck. Since each player only drones at most 16 tiles, lock can be a Euge facture. If you are looking for the perfect tile, the odds are against you. Luck will play a big part in who wins. That isn't so handy.
4) Cloisters. By the time I started playing Carcassonne, back in the mist of time, the first two expansions were already out and I usually played with those. With a bigger tile mix and more ways to score points, cloisters didn't seem that extraordinary. However, with just the original set, cloisters suddenly become strikingly powerful. Drawing a cloister in vanilla Carcassonne can be big.
(Yeah, I know that that's just a subsection of point number three, walk. However, it was still striking)
I don't know if Carcassonne is suitable for high stakes tournament. However, it was fun for a light, casual tournament. After all of these years, even the vanilla version holds up well.