Because then it wouldn't be Legacy? Legacy is in part defined by the tension between having extremely efficient mana fixing and extremely efficient answers to greedy mana bases. Without og duals the balance swings wildly in favor of the efficient answers and will punish non-mono colored decks.
In addition, many current archetypes that people enjoy playing would be destroyed or severely crippled under your proposed banned list: High Tide (Candles/Spiral), Lands (Tabernacle), Miracle Combo (Helm), Storm (LED), Stiflenought (Dreadnought), Aluren, Lots of combo decks (City of Traitors), Enchantress (Sanctum), Elves (Cradle) etc.. It wouldn't be the format that us Legacy players love anymore.
You could do that, but it wouldn't be legacy. It would piss off both players (who would suddenly be unable to play some of their best and most expensive cards), and collectors (who would lose enormous value in their collections). At the end of the day, the value of most reserved list cards is still driven by PLAYERS.
It's a fine kitchen-table rule if you want, but would be idiotic for Wizard's to do, so they won't. Instead, they created Modern.
I assume you're mainly talking about dual lands. How expensive do you think fetch lands would get if dual lands were banned? Better ban them too, even though they're not on the reserved list. And then ban what gets expensive after fetch lands are banned.
People are going to play the best cards en masse and they're going to be more expensive than other cards, especially if they're printed in a very limited batch because of mythic rarity or because it's an old card in a small set in two languages.
You're addressing the symptom rather than the cause. WotC needs to get over its Chronicles posttraumatic stress disorder inflicted by investors--as well as recognize the importance to their bottom line of players knowing that their older cards will always have a format, to complement Standard players buying boxes of new cards--and print cards so players can play the game.
I get as excited as anyone else about the possibility of getting a card in a pack that will be really desirable and valuable in years, but I have no problem with a $60 card of mine dropping to $40 because of a reprint. It can be done without a Chronicles-scale reprint. I'm also not arguing that all decks and cards should be equally attainable. No one should feel entitled to play a deck with three Jace, the Mind Sculptors and four Underground Seas, when they can have a ton of fun and success playing Affinity, Dredge, or whatever. My issue is that WotC could help the best format be even bigger if they practiced logical thinking with regards to the printing of cards, especially for important staples, both reserved list and not. It's not a banned list issue for me.