Historic Challenge is honestly a weird duck. It's one of the only event types with 8 win rewards, which makes its reward track longer than everyone else's - and awkwardly, that UI element starts to break down if you try to cram it too full of stuff. The external link lets us list all the rewards without breaking the interface, and also list all the different mixed packs in the rewards, and also mention the reward that everyone gets on entry (yet another thing that's weirdly unique to Historic Challenge, compared to other events at its level)... Basically, up until now, that link has been doing just enough to skate by despite the event not looking great.
Well, no longer! We're reconfiguring the structure so it can fit cleanly in the client, and you'll see the new version during Strixhaven.