Why is "character diversity" bot a good thing?

Historically, balance efforts have been around certain lanes or positions. For example, there were many champions that had unhealthy patterns coming out of the jungle eg those with strong early lane presence or global pressure. Similarly, there were balance efforts to remove some champions from top lane (though examples escape me right now). So my question: why is it just a given that we should have a wide amount of character diversity in the bot lane? Obviously, nothing is getting reverted, and I am not even complaining (I don't play bot lane). I'm mostly just curious about this from a design/balance perspective. It seems to run a little counter to how the game was balanced many years ago.
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