There could be several answers to that question. What comes to mind is the reliance that country has on foreign aid. It may sound harsh, but the more poverty the more foreign aid can be requested by the corrupt leadership in Haiti.
Another reason, an aquaponic waste water treatment facility would require an infrastructure investment for construction, operation, design and management. I also imagine a lab would be needed, staffed by chemists and bio techs for proper testing, to check for pathogens, monitor PH, ammonia, nitrates, nitrites and bacteria levels, etc.
There could a cultural resistance. In Europe during the mini ice age many people starved because they relied only on grains for food. Many refused to grow potatoes and other alternate crops because of superstitions and an inability to accept change.
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