1. Forward motion: Tropical systems with a fast forward motion (greater than 10 knots) are easier to predict than slow moving tropical systems (less than 7 knots). Fast motion implies a strong and steady path for the storm to follow. Slow moving tropical systems are more likely to do loops, stall and then start moving in a totally different direction. They tend to be near the same place longer causing multiple influences on their motion over long time periods. Fast moving tropical systems are more likely to move in a straight line. Slow movers tend to wobble around more.
2. Lack of data: Compared to land, there is less data gathered over the open oceans. Ocean temperatures and atmospheric influences are more difficult to detect in the open seas. Much of the detection is done by satellites and buoys.
3. Long time: Tropical systems are often in the ocean seas several days. The further out in time one goes, the more difficult it is to predict the future status of a storm. Beyond 3 days forecasts are unreliable.