The trend you are noticing is real, and is not specific to gaming. Kids books, for example, also saw a huge shift from pulp printing of multiple lines to a few major lines. Specifically: multiple high-book-count series like Goosebumps, Baby Sitters Club, Animorphs, etc, all went down, and the industry circled the wagons around huge brands like Harry Potter. It's a shift across lots of genres.
Broadly, it's a shift across multiple mediums as well. You can read about the same trend in movies; there are less "mid budget" movies now, with studios putting out either high budget blockbusters or indie and/or Oscar stuff on the lower budget side. The video game market also has less stratification; you have really big AAA games and lots and lots of small indie publishers, with less in between.
You can find research into this independently for each of the mediums I mentioned above. At a broad level, I blame technology and the internet. Generally speaking, a lower barrier to entry in these markets leads to centralization of product lines. The large industry leaders maximize profit on high quantity products by both dominating distribution and optimizing economy of scale. Meanwhile, a low cost of entry and high number of "hobbyists" lead to a larger number of independents left to scrape up what's left of the market. The end result is what you describe.