Like every single other cool hobby. People stop listening to certain types of music because it becomes too "mainstream". People have even stopped playing 40k tabletop because it was too popular, and are now starting it up again.
Thus is the curse of the Niche hobby.
The difference being that Games Workshop is considered akin to the "Microsoft of the Tabletop Wargaming World" to a large majority of wargamers (with disproportionate pricing to match), and as such there will always be large number (I know a friend who insists it's a silent majority) of self-styled Real Gamers who would never,
ever consider anything GW to be Cool ever again (even those of them who started off with Traveller, or one of the other early adoptions).
Obviously, it's a big 'standard', (compared with all those other numerous gaming systems, e.g. DBA/DBM, and the nearly infinite number of scenarios from prehistory, through ancient and modern history to modern day and, wherever you might place them fantasy-past, present and futures of various kinds technological and/or magical) and the shops are not so much the "little pokey bookshop" kind that you might go to for reasonably priced flock or stranded grass, 'lichen', 1/300th scale waterloo figures, 1/25th WWII vehicles, role-playing guides and whatavyer. Which is yet another reason to compare them to MS.
Personally, I play 40K, but they're not my figures. I turn up every week and we may be playing a 40K campaign, of sorts (in 'our' universe, the Squats are still around), or then again it could be one of the many other systems. Every now and then we try a homegrown WWI trench warfare game, and every time it ends up as a "slaughtered in our hundreds of thousands" scenario, regardless of how we rijig the rules... which probably proves something about the nature of attacking "their trench" by climbing out of "your trench" and advancing across No Man's Land. But most games are not as predictable as that.