There’s one good reason I can think of why one might not want to be a donor.
Instead of having your body put into a coffin as worm food, you can also choose to have it put in the fridge instead, in hopes that future medicine will be able (and future politics will be willing) to fix it up and resuscitate you. To that end it would be nice if your body would still be complete so you can continue to use it.
On the other hand, if you have enough confidence in the future of medicine, you can also give your organs away anyway, and then get cryo’d up. Surely at some point they must solve this donorship sparsity issue with artificial kidneys or cloned livers, and then they can plant one into you again.