Though I have never smoked pot in my life (earnest truth), there are several reasons for the deluge of nomenclature for the drug of preference.
1. It's a linguistic way to get around the authorities. If you're in the market for weed and you are silly enough to be openly selling weed then you will be more likely to be caught for your legal transgressions. At some point, these linguistic derivatives were code names (unknown to the authorities as synonyms for pot).
But since the law enforcement tends to have its ears pinned to the proverbial ground, they shrewdly learn the jargon du jour
and then a new name is given to the product to once again help the dealers to allude the law enforcement.
2. There is an entire culture revolving around drugs, drug use, etc... with niche cultures for each category class of drug.
3. I am a fanatic of literature and language. I rather like having a choice of hundreds of thousands of words rather then a bland option of one or two. Keeps life interesting (at least at a superficial level....)