B6's schedule is unreliable. For example, they just dumped most of their very recently announced SFO-CUN nonstops, going from 14x per week to 2x per week. Only rebooking option I was offered for a Dec. 29 departure was two days earlier. Refunded and booked on AS at a much higher cost.
Basically anything you book right now on either B6 or UA for March is just a placeholder.