The problem is that the Polynesians appear to have sailed thousands of miles across the Pacific going the other direction. In Heyerdahl's day this was hard to explain because the Polynesians would have had to sail against the prevailing winds and currents. The problem for Heyerdahl's theory is that archaeological, linguistic, and genetic evidence indicates that Polynesia was pretty much settled west to east.
One thing we know now is that it is possible to sail Polynesian voyaging canoes pretty much any direction you want to go, at least over long distances. The Polynesian Voyaging Society has sailed Hōkūleʻa [1][2] all over Polynesia at this point and in fact they've sailed her all the way to the North Atlantic since then.
Back in Kon Tiki's day, the performance of Polynesian voyaging canoes wasn't well understood. They can't tack near as well as a modern sailboat, but they can tack to 45 degrees off the wind. And although it's true that prevailing winds in the South Pacific blow east to west, they still vary quite a lot in their exact direction. This means it's possible to sail to an island east of your starting point in a voyaging canoe even though the prevailing winds and currents are against you. Also it turns out the the prevailing winds can completely change directions during El Niño years, which happen with some frequency, making west to east sailing a lot easier. [3]
One thing we know now is that it is possible to sail Polynesian voyaging canoes pretty much any direction you want to go, at least over long distances. The Polynesian Voyaging Society has sailed Hōkūleʻa [1][2] all over Polynesia at this point and in fact they've sailed her all the way to the North Atlantic since then.
Back in Kon Tiki's day, the performance of Polynesian voyaging canoes wasn't well understood. They can't tack near as well as a modern sailboat, but they can tack to 45 degrees off the wind. And although it's true that prevailing winds in the South Pacific blow east to west, they still vary quite a lot in their exact direction. This means it's possible to sail to an island east of your starting point in a voyaging canoe even though the prevailing winds and currents are against you. Also it turns out the the prevailing winds can completely change directions during El Niño years, which happen with some frequency, making west to east sailing a lot easier. [3]
[1] https://en.wikipedia.org/wiki/Hokulea
[2] http://www.hokulea.com/
[3] I don't have a good citation for this last paragraph, but it might be from Ben Finney's paper "Anomalous Westerlies, El Niño, and the Colonization of Polynesia", https://www.jstor.org/stable/677659?seq=1#page_scan_tab_cont...