Real answer? Be because each country can decided to use and divide their top level domain code however they want.
Originally, there were only 7 top level domain suffixes – .com, .edu, .org, .net, .gov, .mil, and .int
After the forecasted success and exponential growth of the internet, the TLDs were expanded to include the ISO two letter country code for every country, and delegated ownership to those countries (or some appointed representative organization for that country).
So, really, .co.uk exists because that’s what the UK wanted to use, and .ba.ca exists because that’s how Canada wanted to use their TLD.
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