There could be many reasons:
** meeting strict emissions requirements are easier with a smaller (e.g. V6) turbo
** R & D costs are recovered quicker because a V6 can be used in many different models/platforms compared to a big/heavy V8
** Apart from the noise and maybe a bit extra long-term reliability (this is highly subjective), there`s nothing that a V8 can do better than a newer well designed V6 (more power, more torque, better emissions, smoother running etc... )
** With a V6 platform, it`s more simpler to offer a hybrid powertrain