Yeah, people on clip shows and documentaries often churn this one out. Not quite true.
Most musical genres tend to have a peak commercial success of roughly 2 years. The hair metal scene - which was always bigger in the USA than here - had been going on since 1981.
As happens it was time for a new generation to find their own thing. Gunge wasn't the only 'new' genre making a stir in the early 90's but no-one blames New Metal, or pop-punk for killing hair metal.
Not too many hair metal bands changed to a gunge style and became successful, off the top of my head I can only think of Pantera, and Alice in Chains.