The third iterations of games always underperform compared to the main releases. I also think that the release dates are a factor.
Red/Green: February 1996
Blue: October 1996
Yellow: September 1998
Gold/Silver: Nov 1999
Crystal: December 2000
Ruby/Sapphire: November 2002
Emerald: September 2004
Diamond/Pearl: September 2006
Platinum: September 2008
I think Crystal came out so soon after Gold/Silver that less people were interested in buying a slightly different version of a game they just bought last year. There was a longer wait the other games so I think more people were willing to buy them
Didnāt play 'em so canāt comment on what they are like but for me personally, the originals are so good that I didnāt have the desire to bother with remakes. Iāve never really enjoyed the inevitable move away from the sprite-based games to really rather average-looking polygons tbh.
Remakes like FRLG and HGSS worked so well imo because they were pretty much the same and didnāt meddle with the plots. The extra stuff was in addition to the original content. Plus the retention of sprites and 2D just felt like an upgrade (going from GB > GBA > DS hardware capability) rather than a change or re-imagining. For the record, I didnāt even bother with ORAS, as good as people say they are, because I donāt play the originals and think, ādamn, these really need a remakeā.
What I would say is that I wish the Gen 3 and 4 games had been ported as digital e-shop titles so that gamers have a choice. They did it with Gen 1 and 2 on the 3DS and other GBA games in general (where Gen 3 is concerned) so I really donāt see why they havenāt done it. Surely itās a license to print free money? Some of the most fun I had in recent years was emulating FRLG on an Android-based handheld and playing it all over again for the first time in years. If I could download it officially for the Switch, I would happily pay to buy it properly (even though I still have the physical boxed original as well).