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Because the amount of file size that all of those champions would take would be massive, as well as the time it would take to go back and add EVERY incarnation of EVERY champion.
I mean, those files already exist. They're not adding anything. They're making them accessible. Not to mention that file size is basically irrelevant, sans all but the HUGEST updates. The reworks constitute maybe a couple dozen champs worth of information, that could be phased in gradually with the addition of the sandbox.