Because their connection is flimsy; a one-off reference from one to the other is not exactly conducive to suggesting a deeper relationship other than acquaintances. Janna is a public activist who wants to clean up Zaun, or at least better its environmental condition, so it stands to reason many people, not just Ekko, might know of her. Granted, people hardly need a reason to rationalize relationships between fictional characters; to each their own.
Also, according to old lore Janna is something of a celebrity, not to mention running around in little more than lingerie; any reaction Ekko might have for meeting her in person is bound to be awkward.
Also, also, fans are starved for lore, so many find it a much more pressing concern to discuss lore in general rather than specific points. Some people may be talking about Janna x Ekko, but any discussion of that nature is largely overshadowed by cries of discontent.
Sincerely,
A Superb Villain
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