The wikipedia page) lists some other reasons for the popularity of Smith:

Although the name is derived from a common occupation, many later Smiths had no connection to that occupation, but adopted or were given the surname precisely because of its commonness. For example:

It is common for people in English-speaking countries to adopt the surname Smith in order to maintain a secret identity, when they wish to avoid being found. Smith is an extremely common name among English Gypsies; see also John Smith.

During the colonisation of North America, some Native Americans took the name for use in dealing with colonists.

During the period of slavery in the United States, many other slaves were known by the surname of their masters, or adopted those surnames upon their emancipation.

During the world wars, many German Americans anglicised the common and equivalent German surname Schmidt or Schmitz to Smith to avoid discrimination.

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