A typical firm website says nearly the same thing as every other firm in town: tax preparation, bookkeeping, payroll, consulting. Google has little reason to prefer one over the other, so the firm shows up for its brand and almost nothing else. Meanwhile the people who would become ideal clients search with detail. They mention an entity type, a software package, a life event or a problem with a notice they received.
Directories and national brands crowd the first page
For broad phrases like "accountant" or "tax preparer", the results are filled with review sites, software vendors offering filing help and franchise chains. An independent firm rarely beats them head on. It wins by being the clearest answer for narrower searches in its area and its niche, where those large sites have thin, generic pages.
Capacity is part of the strategy
A partner who is already turning away individual returns in spring does not need more of them. SEO for a CPA should be pointed at the work you want to grow, such as monthly accounting for small businesses, fractional controller support or tax planning for owners, and away from requests you would decline.