There are marketing reasons for law firm mergers. Law firms make money by having the biggest/richest clients who pay multimillion legal bills. The idea is that these clients want to hire the "best" law firms, and they determine the "best" by looking at which firms are the biggest/richest/hired by other big clients. This logic says that if your competitor firm becomes larger, you need to get larger too in order to appear competitive to clients.
In reality clients are probably smart enough to realize bigger != better, but it's not just pure cargo cult thinking that leads to firms trying to stay large.
Compare this with the world of advertising, where you have three major holding companies controlling most of the market, but each company runs dozens - if not hundreds - of independent agencies.
In reality clients are probably smart enough to realize bigger != better, but it's not just pure cargo cult thinking that leads to firms trying to stay large.