Because if you are ignorant, uneducated gang member who might have gotten past the first grade before dropping out of school, you can't pronounce the full word "whore", you can't form all your letters because you are full of sh** or maybe dope, so you shorten it to "who". The W is silent of course so then the word becomes "ho". People in the US who can actually speak English do not relate the word ho to whore and wouldn't know the relationship if it wasn't for TV and people like Imus trying to imitate gangsters because they think it is cool to imitate killers and thugs, so they adopt the mannerisms and speech idiosyncrasies of these same hoodlums and killers.
Merry Christmas! Ho! Ho! Ho! Meeerrry Christmas!