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In the SI unit prefixes, there's a general pattern of using uppercase prefixes for multipliers larger than 1 and lower case for prefixes that are smaller than one. However, this is not a universal trend. k-, in particular, is lower case, as are h- and da-. In a system with very clear capitalization rules, these stand out as outliers.
What was the rationale behind this capitalization?
The decision that k is lowercase was made in the early days of the cgs system, long before the SI system. The available prefixes were milli up to myria.
In 1879 the International Committee for Weights and Measures, CIPM, adopted the proposal to abbreviate only in lowercase, because a mixture with capitals would be sensitive to mistakes and reduce the ease of writing. The least used prefixes deca/deka and myria, having the same initial letters as deci and milli, were not abbreviated because they would require capitals or an extra letter. The member states of CIPM started using these official abbreviations. The new prefix mega (1874) was not considered because it was rarely used and unimportant for commerce and trade. Higher prefixes did not exist.
