Well, oversaturation and lack of commercial success are examples of why the movement ended. When it came to lack of commercial success, it happened to a lot of great bands. Diamond Head, Tygers of Pan Tang, Tokyo Blade, Quartz, Raven, Samson, Blitzkrieg, Tank, Saxon (very well-known underground but not in the mainstream), Grim Reaper, etc. I think what had a lot to do with the success of some bands like Iron Maiden, Motorhead and Def Leppard (to name a couple) was they either went commercial in just the right way (Pyromania in 1983 and Judas Priest with British Steel), stuck to their guns and became the definitive band of the movement (Iron Maiden), or they helped revolutionize metal a bit before the '80s (Motorhead with their eclectic, ever-modifying musical influence and crossover appeal between punk and metal fans. Also, Judas Priest).
The trends that occur with many of the bands I previously listed is that they eventually stopped making albums after a few well-regarded releases and faded into obscurity (Grim Reaper, Quartz, Angel Witch, Satan, Blitzkrieg, Pagan Altar, Witchfinder General etc.) or they end up watering down their sound, destroying their credibility and whatever fanbase they previously possessed (Diamond Head's Canterbury, Raven's Stay Hard, Tygers of Pan Tang's Crazy Nights, Tokyo Blade's Blackhearts and Jaded Spades and many more). Some made comebacks like Raven did and some like Saxon and Satan (in the form of Pariah) went well into the '90s and the '00s and some just stayed underground and made their own success like Venom and Saxon. Some even made new albums in this decade after decades of silence. Regardless, the movement was dead long after.
I also think that when bands out of the U.K. like Mercyful Fate, Bathory, Megadeth, Riot, Jag Panzer, Helstar, Liege Lord, Omen, Helloween, Sodom, Death, Morbid Angel, Possessed, Coroner, Metallica, Saint Vitus, Candlemass, Pentagram, Trouble, etc. were inspired and began making further modifications to the revolutionary sound of the NWOBHM (the virtuosity of '70s rock and metal mixed with punk's velocity and attitude), that's when power metal, black metal, thrash metal, death metal and doom metal were created, the "NWOBHM" label lost relevance and became a revered accolade to that old guard who made a monumental change in rock music and provided an origin for many of the other genres to be created.