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Releasing them on VC would also make a challenge arise of making potentially those pokémon transferrable to the newer games. Having those VC PKMN games out on the digital market with their old set of Pokémon which would be clustered out from newer games would not make a whole lot of sense commercially speaking IMHO, and making them linkable with the newer games would be a challenge as PKMN data has drastically transformed from Gen III onward. First gen didn't really have IV the way we have it now, no genders, learnsets were different, etc... Things have mostly settled for the way they are now during Gens III and IV.
Whereas the remake way, not only makes the games more appealing for people to purchase because they look better and take advantage of the new hardware, but also because they reincorporate the newer mechanics, thus making them connectable with the newer games since the PKMN dataset is therefore the same.
As much as I'd like the old game to be made playable on newer hardware and for them to work out a way to import pokémon trained in the old, unaltered games into the newer games, i can totally understand why they would not go that way.