It isn't just banks, and this phenomenon does not necessarily have anything to do with banking as such. You could draw very similar-looking diagrams for:

-the airline industry

-the railroad industry

-the media

-the beer brewing industry (microbrews excepted)

In fact, I invite anyone to do so. If there is anything these cases have in common, they are fields with low profit margins where economies of scale give a large competitive advantage; thus, centralization is favored. The end-game, of course, is when each industry has only one player left standing. In the case of the long-distance bus industry in the USA, that moment arrived years ago, when Continental Trailways ceased operations, leaving Greyhound as the sole remaining operator. I expect the railroad and airline industries to reach that point in another 20-30 years; the banking industry is actually not as far advanced as these two.