Personally I wouldn't care what gender the protagonist in the game trailer is. I'd be more focused on the quality of gameplay. It's definitely possible for Bethesda to flip the script and indeed showcase a female protagonist but I highly doubt it.
Unfortunately, It's not only a preconceived notion that a man on the cover sells more copies. It's a proven statistical fact. According to sales history and marketing research.
Ken Levine spoke on this topic when fans questioned his decision to feature another 'white dude with a gun' on the cover art for Bioshock Infinite. Even though Ken held a contest allowing fans to choose the reversible cover art, Ken said that after all the hard work that went into creating the game, he was not going to risk potential sales to new fans by using cover art that didn't specifically target the focused demographic which is male gamers.
Even though there are many devoted female gamers who are far from casual players, marketing research always determines what target group to appeal to when you want to make as many sales as possible. Bethesda is a business above anything else. They are simply doing what any business that wants to make max profit would do. Appeal to the more common consumer demographic. Which is video games are male gamers.
Many take a political or ethical stance on it, but at the end of the day, it's strictly business. You're more likely to see a female protagonist being featured in a trailer from a game where you chose your gender, if that game was a new IP.