Somebody with "deep knowledge" please explain to me. I see many posts like this where someone is waitlisted for a Miles + $ upgrade. I don't get it. Every time I try Miles + $ I am upgraded on the spot. I just did it for a SFO/BKK in January. No waitlist. Click, CC and done. I've done this a dozen times or more and never saw a waitlist.

So why do some flights get waitlisted on Miles + $ and others not? Is it the flight? Status?