Yeah, I was told it was because of the risk of blood clots - which increases *after* surgery as you spend days not moving around very much. (Brassard did try to get us up and moving as soon and as often as possible, but it still wasn't anywhere near what the average able-bodied person does in a day, and that's even assuming everyone was capable of following his ideal guidelines.) The more invasive the surgery and the more immobile you'll be after it, the higher the risk of throwing a clot.
I have a (cis) friend who almost died of a pulmonary embolism without having had any surgery at all, so I was happy to be more safe than sorry...