I know from the Wikipedia that the slang “what’s your twenty”comes from the term ‘10-20’ of CB slang. But I don’t understand why people used the code ‘10-20’ but not something like ‘5-15’?
Ten-codes used to represent common phrases in voice communication were developed during the 1930s by the Association of Public-Safety Communications Officials-International and have been widely used by police forces in the USA.
Ten-codes first reached public recognition in the mid- to late-1950s through police-based television series and were adopted by CB radio enthusiasts.
There are variations in the codes used.
The use of 'Ten-Codes' was to standardise commonly used phrases across radio communication. The 10-20 code is 'Location'. "What is your 10-20?" means, what is your location? This evolved to, "What's your twenty?" (the ten was commonly dropped for the more frequent codes).
The reason 'why' people use it now can be, because it is different or novel or because it offers a method of communication that is typically only understood within a particular group.
"What's your twenty?" was probably only used by serving or former police or military personnel when the 'ten codes' were first issued.
When slang becomes popular enough, it stops being slang and becomes part of the language. "What's your twenty?" is now used widely enough to be understood by most people.... apart from my mum ;-)
I could send a phone message (SMS, TEXT etc.) to some of my friends asking, 'What's your 20?' and they would understand it meant, where are you?