Nobody knew the play would go for a TD. So the risk is burning a time out on a mundane play to prevent a penalty that might not have been called anyway. If everybody knew doing so would preserve a TD, they definitely would have called it.

Aside from that, I'm not sure whether Holliday has authority (from Bronco) to call a timeout. I don't know how that works -- whether any coach is allowed to call a timeout at any given time or what.