What killed the NWOBHM?

NWOBHM was not a sub-genre. An era, a movement maybe, so it didn't died, simply evolved. Some bands stayed (Iron Maiden, Saxon, Venom...) continuously, some (Jaguar, Quartz, Battleaxe...) occasionally, and many either passed away, and at least one change dramatically (Def Leppard).

Many bands try today to capture the old feeling, like fashion does, likewise music. . Look all the retro bands playing in the 70's feeling/vibe, and thrash bands trying to sound like the ones in the Bay area at the end of the 80's. I see no fault, nor problem.
 
The success of glam metal and pop metal as well as fans moving to thrash metal and power metal.. There were British bands who followed those genres of metal. This is where bands such as Onslaught, Dogs D'Amour, The Quireboys, Wildhearts, Wolfsbane, Bolt Thrower, etc ended up forming.
 
The New Wave of British Heavy Metal was not a style, it was a movement. Musical movements, by their nature, aren't really mortal, rather they dissipate leaving only their legacy. The period between 1975 and 1983 or so saw the revolution of heavy metal into something more distinct and specific, while at the same time more diverse. Iron Maiden, Saxon, Motörhead, Angel Witch etc. influenced extreme metal. After the movement had spawned its ultimate legacy, that is, extreme metal, it became static, no longer an active movement.
 
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One of the factors is that the generation of fans for it grew up, got jobs, mortgage, married and stopped actively following it. As the next generations arrived they wanted their stamp on it. Then there was a who can play fastest, loudest and have the most horrific sounding album titles, as it evolved. A mixture of other factors also. I was into Rush and Zeppelin and other prog rock bands in the day, but I bought the motorhead albums, ace of spades was superb and still is. I remember buying Saxons wheels of steel album and that was just great in the day. I used to go seeing a couple of bands a month.