A recent survey of accounting firm partners from the CPA Consultants' Alliance found that over half of respondents (51.7%) said procrastination or denial was a primary cause for firms' succession troubles. Another 48.3% of respondents also said that "Lack of future leadership talent" was a primary cause of succession issues.
So, some firms have better things to do, some firms are sticking their heads in the sand and some firms lack the "bench strength."
Here's the full chart from the NJCPA:
It bears repeating that the "lack" of talent is misleading. There's talent everywhere, as the enrollment and graduation numbers from the past decade have proven. it just so happens that all this talent have found opportunities that are much, much better than working for an accounting firm that has had the same business model for 30 years, has only one career path and has questionable prospects for the future.
The "leadership talent" that these firms want does not exist. It's a figment of their imagination. As soon as the current leadership accepts that, they can start making their firms relevant again and then maybe some potential successors will start appearing.