There’s more to it than concrete construction w/o rebar, or even concrete construction with concrete diluted with sand.
First the fault ran right under Port au Prince, only the most advanced earthquake-proofing technologies are any good under those conditions (think about the bridge the Japanese built over the earthquake fault with its computer-directed stabilizing motors). Second, one has to explain why a poor country was using concrete, rather than wood. I suspect it has to do with the same foolish land-use policies under the Duvaliers that destroyed Haitian agriculture and made the place subject to landslides and flooding.