It's important to consider how the water used for livestock and fish will be treated and disposed.
The first priority should be fish. If you can get them to self-sustain in a pond or other reservoir, or even buy them from local sources, do it.
Livestock are only necessary where they aren't practical to raise, or where the local terrain and climate makes it impossible. Sheep are really very useful on some islands (but not Hawaii), and I'm sure somewhere, they work.
If you aren't worried about the livestock, then they don't need to be considered. If you are concerned about the effect of a deadzone on fisheries (if you don't have a huge area of open water nearby) then it's a good idea to consider the livestock. There is no such thing as a perfect balance, so you have to weigh your options.
As for your question... I have a feeling that if you were doing this project for the first time and had no experience, you would have missed that factor entirely. I'm not saying that anyone has to make one decision, or that one option is clearly best. It's really up to you to decide what factors are important to you. For a few years, I did what the majority of the islanders were doing: live on the ocean, eat fish. I can still do it, but I'm still interested in making decisions that protect the resources and environment.
As for why some people aren't trying to raise livestock... you would have to ask them. Some people don't like to raise animals. Some people aren't interested in making money from farming. Some people like to use their land as nature reserve.
in most areas of agriculture there are not enough farmers to satisfy the demand and in areas where it's not a big issue to consider livestock or fish raising.
The reason for considering some of the factors is to avoid making the wrong decisions and to make good ones. And yes, it's important to consider some of the factors because they're important.
