If you look at a map of the South Pacific, you'll notice there is a vast archipelago stretching between the Malaysian peninsula, through present day Indonesia and the Philippines, eastward towards Fiji. This made for relatively short and easy island hopping voyages, as ancient mariners didn't have to cross thousands of miles of open ocean.

Take a look at the Atlantic. There are about 2800 miles of empty water between the Canary Islands and the Caribbean. This took Columbus about 5 weeks to cross and made for a much more formidable obstacle to early sailors. Of note, it's a greater distance than even Polynesian voyagers crossed regularly - from Tahiti to Hawaii, for example, is about 2600 miles.

Winds and currents also made crossing the Atlantic tricky -- the Gulf Stream, for example, flows to the Northeast and trade winds and currents around the Canaries tend to push ships southwest.