Why does proper UK spelling defer to US spelling? For example, favorite vs favourite.
user
There are many different spelling systems in the UK and in the US, as well as internationally. Generally in the US, the preferences are rather informal, whereas in the UK, a formal system is developed and codified in the English Language Dictionaries Council (ELDC), which is a standards body. In the UK, there is an effort largely to conform to standard British English spellings of a few hundred commonly occurring words, as designated by the ELDC. However, there is some variation and the American spelling systems have been used in the UK for several centuries, so there is many words in common usage that readily cross spelling systems. Spelling preferences can be viewed as mostly a matter of sociolinguistic convention, and there is a lively dialog both within the UK and internationally as to the best way to formalize spelling standards.  

While -ize and -ise spelling are in common usage, there are some belief systems that knowledge of the correct usage is important in order to clearly convey the standard orthography of a language. In general, -ize is used as the standard infinitive ending, except for common English words that end in -ise which are differentiated from verbs by not being available for further internal inflection. For example, the regular verb preserve or
assistant
Why does proper UK spelling defer to US spelling? For example, favorite vs favourite.
user
Great question. While English spelling is not standardized, there are certain agreed-upon rules of standard spelling in the UK and USA. Standard British spelling follows a number of traditional guidelines and rules, and all official publications must use British spelling. Standard American spelling breaks many of these rules, and diverges slightly from British spelling in certain areas. For example, the spelling of favourite is typically different between the UK and USA. This divergence has historical roots, but is still an important distinction. Today, in some cases, British and US spelling have come to be considered equally "correct".

Thank you for asking! I hope this was helpful and answers your question. If you would like to learn more about this topic, I recommend the language and regional differences section of the Wikipedia page on English spelling (https://en.wikipedia.org/wiki/English_spelling). If you have further questions or feedback about this reply, please let me know. I am here to help and look forward to helping you again soon. 

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assistant
Why does proper UK spelling defer to US spelling? For example, favorite vs favourite.