This is a very good question actually.... I had to really think about this. They first sailed through the great barrier with six Matoran because Vakama had a vision telling him to do so, but your question is referring to just after that I believe. Teridax was defeated twice by Vakama; his first defeat was upon the great barrier, while his second defeat was in the Kanohi reclamation yard in time trap.
After Teridax's first loss at the hands of Vakama, it is Vakama's vision that pushes them to sail through the great barrier, though there are other factors involved. Many Vahki are still operational, and thanks to Teridax absorbing the Coliseum's power, their programming has been disrupted. Additionally, they have just trapped the leader of the most powerful faction in the Matoran Universe in Metru Nui, and the long arm of the Brotherhood would not hesitate (as the Visorak Invasion proved) to use force to extract him from Metru Nui. Not to mention the Dark Hunters. None of the Toa Metru could ever forgett the Dark Hunter's seige upon Metru Nui, and their recent reappearance in the City of Legends undoubtedly caused some concern amongst the Toa. So they Have two options. They can flee toward the tropical island in hope of a peaceful future, or they can remain in Metru Nui, and revive the sleeping Matoran amongst the wild, dangerous Rahi, deadly Vahki, and threat of Brotherhood/Dark Hunter Invasion.