You overestimate LoZ's popularity and underestimate Mario Kart's. Not even those two particular entries, but in general. And I say that despite being a sucker for both the LoZ series and the less conventional BotW/TotK.

BotW sold a lot for a Zelda game and even then it sold a lot less than Mario Kart 8 DX (despite it being a port of an older game).

My take though? I got the bundle mostly because I expected better from MK World. It's not a complete dud, but I would agree it's rather underwhelming. MK8 is still my favourite and I would never consider it "a boring racing game" in any way.