At least of late, Nicaragua has been having a rebel/political conflict making it a very dangerous place to be. So collecting, or even research would not be a priority. Local people are using readily available grenades to fish for their dinner, making just being in or near the water in the lakes risky.
This along with many of the more stable countries of the area becoming more and more environmentally conscious, and not wanting to have their endemic plants and animals removed by foreigners for free, on principal.
We all realize cichlids are a main part of a staple diet for people, and this of course puts much more pressure fish populations than a collector or two, but feeding local populations is looked at differently than interlopers coming in a carpetbagging resources.