It happens to a lot of games, really. Fortunately the internet has made it more difficult for games to fade and be forgotten since their fanbases tend to keep them alive through mods, forums, youtube videos, FAQS, etc. The Earth 2150 series is an example: the first few games were great and innovative during their time (only Warzone 2100 had done that type of design-deploy-destroy before them), the graphics were great, and the campaign and plot certainly were different from the norm. However after the mediocre Earth 2160 was released, interest in the game just died outright.
The average RTS player, even some of the experienced ones may have absolutely no idea the game ever existed. Yet, it's still a fond memory for some of us, and some devout fans are still trying to support it with mods, balance patches, translation fixes, etc.
Avvie-free for life!
Nowadays any game with blockbuster sales (a whole category above just 'a lot of sales' - we're talking at least several million copies in the first week or so) is virtually guaranteed a followup unless the publisher or developer somehow dies right afterward.
Games that just sell 'a lot' (several hundred k to one million in the same period as above) may end up disappearing and games that are niche from day one are likely to be one-offs, not to mention the fact that the developers who make these kinds of games often prefer to spend their profits on something new rather than revisiting the same concepts again.
We all know about video game series that are tampered with to the point where a Franchise Killer happens, but return in full form soon after. Others are not able to return this way, but have a dedicated cult of fans organizing petitions to bring it back (and in some cases even making fangames).
But is there any video game series that sells a lot of copies in its day, is run to the ground, and is then completely forgotten despite being well-loved in its day? It would be very sad if this happened to a series.