Seeding a planet with these organics from space doesn’t make life possible on the planet, but it does make life quicker to develop. The organics will have come together in space when the planet was still just a cloud of dust. By the time the planetary environment is ready for organics to survive, they’ve already been raining down for millennia. It saves time, and more importantly, it means that life based on these compounds will develop before anything local gets the chance to evolve. Thus, anywhere that carbon-based life is possible, it will probably be DNA-based, and it will probably be made from G, A, T, and C.