I think (and this is my interpretation so it could be wrong) that the OP means "mass produced" as in produced near the scale of something like Bud or Coors. And I also infer that the OP means in the US (Bass, Guinness, Fullers, and Sam Smiths are all Euro-ales)
If this is what the OP is asking, I would surmise that the dominance of lager beer in the US results primarily from prohibition. Near the turn of the century, there were thousands of breweries, mostly small, producing all manners of ales and lagers. According to beerhistory.com, in 1873 there were 4131 breweries (record number) that produced 9 million barrels of beer. According to the same site, in 1933, there were 31 breweries back in operation.
Post-prohibition US breweries focused primarily on lagers. Because there were so few breweries after prohibition, a relatively small number of breweries rose to dominate the scene relatively quickly, meaning lagers dominated the US beer scene. One of the breweries was Anheiser-Busch, who waited out prohibition by legally selling extract (to lots of people who illegally used it to produce homebrew).
Exactly why these companies focused primarily on lager brewing goes beyond my beer history knowledge. But I did learn the other day (from a Travel Channel show) that another brewery was also operating in St. Louis just before prohibition, and was much larger than Anheiser-Busch, called the Lemp Brewery. The Lemp family brought with them from Germany a recipe(s?) for amazing lager beer, which in the mid-1800s was a fairly novel concept for the US beer market. Just before prohibition, the Lemp dynasty was becoming huge, and was dominating the US beer market b/c of the popularity of their lager beer. Unlike A-B, they assumed prohibition would last, so they called it quits for the brewery and sold it off. So if I were to guess why lagers were the primary focus of the small number of breweries who ran the scene following prohibition, it would have something to do with how well breweries like Lemp had marketed lager beers, and how popular they were becoming across the country.
If that's what you were asking, I just gave you a lot of text for nothing. But at least it was good procrastinating from the real writing I need to do for work.