Basically because the Online game is a reflex of IRL game. There's Standard, there's a World Championship. And there's Expanded, with its rotation both. And HGSS rotated from both, that's all.
Everything else passed by. It's not a business anymore because the money flux comes from new cards, new packs, new decks. There's no business in cards half the players+collectors already own.
Legacy was a huge gift from Arceus for the online only, to give sense to all the cards stored through years that rotated. The format doesn't even exist anywhere else. And the loose of interest of the format comes from no movement. It's always HGSS 'til end B/W. There are 4 or 5 Meta decks and everything else doesn't work as good. So you face same decks once and again, and you play those decks or you loose.
Open Expanded starting HGSS? Not IRL, because the rules of the Company. So also not in here. Would be a query to TPCi. Sadly, forums doesn't work too much to make them arrive our requests.
"Realistic" option would be requesting new formats just for the Online, but experience with Legacy says it's hard to make it work if no movement of cards, and no reflex in IRL.
Edited by Chasista, 20 November 2018 - 03:19 PM.
WE WANT LEGACY TOURNAMENTS BACK !!! If you also want it, show your support http://forums.pokemontcg.com/topic/55966-feedback-making-it-easier-to-get-the-cards-you-want/
5th day reward packs list http://forums.pokemontcg.com/topic/56679-5th-day-log-in-pack-is-random-got-a-gur-dd/?p=593037
Foro en español, también existe: http://forums.pokemontcg.com/forum/2-español/