Much analysis has been written about the prevalence of alpha males in romance literature. Some analysts have assumed that women love alpha males because they've been brainwashed by patriarchal society. Some analysts go for the evolutionary psychology angle: women go for the male who can protect them during childbirth and provide them with healthy offspring. Some analysts translate this into modern terms: women want money and security (doesn't everybody?).
Some of these theories are complimentary to women but far too many of them find a flaw at the heart of women's motivations. Women are sooo....They also miss some basic writing reasons:
2. Alpha males can have complexity.
3. Alpha males are hot but the hotness becomes increasingly irrelevant as the story moves forward.
From a writing point of view, the first two elements are incredibly useful. One, stuff needs to happen in a story. Alpha males--or males who appear alpha--are good at creating conflicts. And they are good at bringing conflicts upon themselves. They are also good at ticking people off and then, if they are good guys and capable of growth, learning and reconciling. See Darcy in all his many manly manifestations. 
Alpha males allow for complexity. Rochester comes across as an alpha, but he gets himself into hot water mostly because he is less alpha than he appears and naive. I've written elsewhere about why he is actually a sweet beta guy trapped in an alpha body struggling to get out.
Alpha males are portrayed as hot (and arguably are). However, interestingly enough, there is a gap between how they are described and how they appear visually. Of the Rochesters, Orson Welles likely comes closest to the book description but consider that the same character has been played by Colin Clive, George C. Scott, Zachary Scott, Timothy Dalton, Charlton Heston, Michael Jayston, Patrick Macnee, Ciarin Hinds, William Hurt, Michael Fassbender, and Toby Stephens.
There is a vague similarity between these actors. But they couldn't pass for brothers.
Consider too that the Hallmark versions of Alpha males from books are often more yuppie handsome than their ruggedly handsome written counterparts. (I generally find Hallmark characters rather dull.) Many romance books make the point that the alpha males are more tough-looking, more Bogart (or Fillion) than Laurence Olivier. Yet Hallmark movies are extremely popular. Arguably, looks aren't the point. Or, rather, the look of Hallmark is the point. Regarding the written romance, the point is the inner life versus the outer life. And the sweet resolution. 
I love the Bingleys but it is important for writers to realize: Bingley is exactly who he appears to be. A conflict can be built around that, but it isn't the same type of conflict that seems to automatically attend an alpha.
