Good Beer Hunting recently covered a nice portion of this information in phenomenal detail (Part 1, Part 2, Part 3). All About Beer has a great article as well, but let's quickly set the scene. Beer distributors form the "middle tier" of the three tier system, helping to move your beer from the supplier (brewery or importer), to the retailer (bar, restaurant, grocery store, etc). Simple in theory. This system was federally established around 1933 when the 21st amendment was signed into law, ending the Great and Noble Experiment of Prohibition. Each state was allowed to regulate the details, including legislation around franchise laws, self-distribution, etc. I won't get into the full history here, and you can read about the background in any of the links above and you'll quickly become an expert. I would highly suggest you do if you're at all interested in that sort of thing.
The Business Model
Most (not all) traditional beer distributors exclusively make money by selling cases of beer to the retailer tier of the industry (bars, restaurants, stores, etc). For the most part, they don't really care what sort of liquid is in the cases. I don't say that to imply that wholesalers don't care about their supplier partners, but a margin is a margin, so if it comes from a case of Bud Light or Brooklyn Lager, it amounts to a chunk of revenue for the wholesaler at the end of the day. The margin might be a little (or noticeably) better on the case of Brooklyn Lager, as it commands craft pricing, but they sell 100x that amount of Bud Light, so in terms of priorities, you can see where they're focused. The suppliers that make them the most money retain the most mindshare, get access to the best distribution channels, and generally command their attention far more effectively than a smaller supplier does. If you want to hear a hilarious and very real anecdote on this topic, listen to this podcast with the famously outspoken founder of Magic Hat Brewing Company Alan Newman...it's really worth listening to the whole thing, as he puts all his cards on the table.
None of this information is revolutionary. This is called "selling beer", and it's been this way for a long time in the United States. Small breweries have dealt with this reality for a long time, and large breweries (the two or three biggest suppliers anyway) like it this way.
Consolidation, expansion, proliferation...