Ethics, and because people who qualify for physician assisted suicide are generally unsuitable for organ donation given that they're terminally ill. The other complication is that organ donation is ideally done from a brain dead person on life support, basically immediately after life support is removed. Prior to that, there's a lot of haggling and negotiation and figuring out which organ goes where, to save precious time. Physician assisted suicide does not legally occur in hospitals (generally), and that process of working out who gets what and logistics and so on doesn't happen. So even in the best case for organs in a physician assisted suicide many hours go by before organs could even be started on their journey to where they need to go, causing the organs to degrade and become less suitable.
The dirty, but poorly kept secret of organ donation is that it isn't actually a good option after death the vast, vast, vast majority of times anyway.
"I can tell you that it's about one percent of all potential donors have organs that are actually suitable for transplantation," Veale notes
A step-by-step transplant explainer.
www.vice.com