So here's a thing.
In XY, you can catch 453 different Pokemon, and (if I counted correctly) you can receive 133 other Pokemon through breeding and catching friend safari Pokemon. For a moment, let's pretend that Hoopa and Volcanion will be obtainable without internet connection.
Now pretend you and a friend live in some area in which your 3DS systems have no internet connection. If you both have a different version, you can both naturally obtain 589 different Pokemon. That's 589/721, which is about 82% of the Pokedex.
Now imagine this same situation a few years back, in which you and your friend have a pair of HGSS games. Assume you can find your way to a Gamestop that can give you Celebi.
With the Safari Zone, Pokewalker, pokegear, and swarms combined, there is one Kanto Pokemon you can't obtain (Mew), 15 Hoenn Pokemon you can't obtain (Wurmple, Silcoon, Cascoon, Beautifly, Dustox, Taillow, Swellow, Nincada, Ninjask, Shedinja, Regice, Regirock, Registeel, Jirachi, Deoxys), and 46 Sinnoh Pokemon you can't obtain (Turtwig, Grotle, Torterra, Chimchar, Monferno, Infernape, Piplup, Prinplup, Empoleon, Starly, Staravia, Staraptor, Cranidos, Ramparados, Shieldon, Bastiodon, Burmy, Wormadam, Mothim, Combee, Vespiquen, Cherubi, Cherrim, Drifloon, Drifblim, Glameow, Purugly, Stunky, Skuntank, Riolu, Lucario, Rotom, Uxie, Mesprit, Azelf, Dialga, Palkia, Heatran, Regigigas, Giratina, Cresselia, Phione, Manaphy, Darkrai, Shaymin, Arceus).
that's 431/493 obtainable Pokemon in HGSS, which is about 87% of the Pokedex.
This means that even though XY has the largest amount of obtainable Pokemon, HGSS was still a more accessable Pokemon game for the time.
