Because most ppl want to have communitary tanks and owerhorns and ommunitary tanks dont get long well despite some people insist on keeping FH along with tank mates and this wouldnt work out.
Flowerhorn are just as aggressive as any central american cichlid. When kept alone just like any other cichlid they show more color and are more dominant. When kept in a community only the largest more dominant fish show all the color. Flowerhorn are said to be more aggressive mainly because most are raised separately from a real small age to show more potiential at a smaller size. Genetics play a role too.