Well, to make Loki's point more pointedly, what's in it for me? As a general rule, I don't much care for people, being that they're generally pretty contemptible, so "saving lives" doesn't do much for me except for a mild chuckle.
I don't like getting stabbed with... well, anything, but sharp metal objects especially, nor do I enjoy taking time out of my day to answer a bunch of very personal questions about my life. I could be incentivized to, of course, and in fact, I sold plasma a few times back in my college days when I needed a little extra money to go out and enjoy myself... but it seems that government and society feel that preventing some moral hazard is more valuable than getting my blood, so in my body is where it'll stay until the cops riddle me with bullet holes, and y'all can have as much of it as you can manage to collect at that time.
On the other hand, I'm on the organ registry, because it's free, involves ticking a box when I get my driver's license, and I don't give a shit what happens to my corpse once I'm no longer using it. So yeah, all about the incentivization. Gimmie a compelling enough reason to go get stabbed and weakened once a week, and I will. Don't, and I won't.