A more robust airframe may have given Gannet the procurement edge

Why did the Royal Navy reject the Blackburn B.54 family in favor of the Fairey Gannet?

Probably because the Fairey design was judged to have the more robust airframe, although the difference may have been quite small.

The Blackburn and Fairey proposals appear to have been closely matched rather than radically different concepts. Airframe robustness is recalled, with some uncertainty, as the likely discriminator in Gannet's favor. Procurement decisions can rest on very small margins when both candidates meet the main requirement. The brief answer points to survivability and service durability as possible deciding factors in a naval-aircraft competition.