My understanding is that it’s related to the decline of three and four engine planes. Originally airlines preferred them over two engined planes because they were safer, had greater range, and could carry more passengers. On the other hand, they were more expensive to operate due to requiring more maintenance and fuel. Additionally, these planes were larger and couldn’t get into places that smaller twinjets could. Eventually technology improved to the point that the (literal) costs of more engines began to outweigh the benefits to airlines. These days most passenger airlines only operate twinjets and the remaining tri- and quadjets are relegated to cargo.