Because the outrigger canoe is the earliest stage in the development of multihull boats, which are generally acknowledged to be more seaworthy than a monohull boat.
Kilopi, you might want to seriously consider that there are important reasons why multihull boats
haven't taken over the oceans, and didn't in ancient times:
1) Smaller, more crowded individual hulls- especially problematic if you're planning to transport significant cargo.
2) Physically, a catamaran is a lot wider. This is a problem when we're talking about large merchant vessels. Remember that we have issues physically fitting all our shipping traffic into our old harbor? Catamarans take up more space.
3)
Holding together a large catamaran hull requires sturdy structural members to cross-link the hulls. While our technology would in principle probably allow such a thing, in reality our shipbuilders are all far more experienced with lightly-built construction styles used on the galleys of the period. They would have a lot of problems with their catamarans breaking apart until they learned better.
And if you remember what I mentioned earlier about
why ancient peoples tended to be technologically conservative, and noting that if you fuck up a ship design and it sinks
dozens of people die...
Well, I hope you can understand that upgrading to catamarans is not going to be some easy thing.
Plus, I hope that you've noticed the pattern that "find an excuse to introduce an ahistorical technology that I'm totally sure will work better than anything anyone tried in real life, solving our problem with This One Weird Trick" is exactly how this quest
doesn't work.
...
The idea of experimenting with catamaran hulls is actually better than the other similar ideas I've heard you come up with, but realistically even if some Greek shipwright thought of it, there'd be a very lengthy experimental period during which time things might go very wrong and kill people.
I will note that there DOES seem to have been some indication that multihull 'ships' (basically giant floating slow platforms) were constructed as fleet flagships for some of the navies of the Hellenic period... but that's 150-200 years in the future from our current point of view.