1-Haiti is insanely poor partially (mostly imo) because after they freed themselves from their enslavers, the rest of the western world gave them the cold shoulder. In fact, from the moment they earned their independence in 1804, they had to PAY REPARATIONS to their former masters, the French, all the way until 1947, on average around 40% of their GDP every single year. Estimates of the total cost paid range from $20-30 BILLION in today's dollars (according to an article I found written in 2021)

2-Global population numbers exploded throughout the 19th and especially the 20th Century. Combine this with the absurd levels of poverty caused by my previous point and there was practically no way for them to make enough suitable infrastructure even if they wanted to. Even if they did, it probably wouldn't have done much good because...

3-The previous two factors alone would be sufficient cause to produce massive casualties, but combine that with the fact that the epicenter of the quake basically hit smack dab in the middle of Haiti's population centers (just inland, next to their capital, Port-au-Prince) meant that they basically suffered THE worst possible consequences given the situation.