Because anyone with experience rowing a canoe is going to wonder how you take one across the ocean without drowning. And the answer is, you use a boat that can handle the ocean.

I initially noted that the Polynesians who settled Hawaii didn't have outriggers, and that was wrong - they did have single outriggers. They maintained heavy use of catamarans.

The technological progression goes:

raft -> catamaran ("This raft would be more stable with a canoe at either end") -> single outrigger ("I don't need the passenger space of the raft or the second canoe, but I still need the stability, so the second canoe will just be a log") -> double outrigger ("I like being lightweight with no passenger space, but I wish I could turn in both directions")