The pattern that has developed for Pokemon generations seems to generally consist of:
• New Generation Pair (Red/Blue, Gold/Silver, Black/White, etc.)
• Third Version / Sequel (Yellow, Crystal, Black 2/White 2, etc.)
• Previous Generation Remakes (FireRed/LeafGreen, HeartGold/SoulSilver, OmegaRuby/AlphaSapphire)
Sometimes in that order, sometimes with the remakes releasing between the new pair and the third version/sequels.
I don't want to buy a new Pokemon game every year. I'd much rather buy one per Pokemon generation, which would probably mean one game every four or five years. So which would be the best choice going forward? The initial releases are the most popular of each generation. The third version is essentially an expanded, better version of the initial releases, so it seems like an obvious choice. But the definitive version of each generation's new games is released some ten years after that generation ends.
So, let's take this generation as an example. Would you have bought X/Y last year because they were the first ever fully 3D Pokemon games? Would you have waited till this year, hoping for ORAS? Now that ORAS is released, would you buy that or wait to find out about X2/Y2/Z? Obviously it would be silly to wait a full year after their release to finally buy ORAS. The community would be a lot less active by then.
The decision is made a lot more difficult by the inconsistencies in recent years. For instance, HeartGold/SoulSilver could probably be said to have been the optimal choice for fourth gen, with Platinum close behind. However, no remakes were ever released in fifth gen, so waiting for them would have meant skipping the whole generation. Additionally, it seems Black & White (the initial releases) are actually more highly-regarded than their sequels. Who would have guessed that the best games of that generation would have been the first ones?
