Klefki shuts down your Manaphys Ability, so if you run it, a Lost Zone Box opponent could Boss it at some point and use Moonlight Shuriken to KO two of your Pokmon (if nothing else, they can KO Klefki and Manaphy themselves)
Its cover card, D/D/D Dragon King Pendragon is a great card, with an easy to special summon condition and the ability to destroy Spell and Trap cards while boosting up its already great Attack stat
Perhaps the answer lies in its extremely annoying Pokemon Power
The release of Lost Caverns of Ixalan featured a normal amount of variants, with Neon Ink being the ultimate chase, while adding Universes Beyond Jurassic World and Special Guest Magic: The Gathering cards that are all worth money as singles
If you circled any of the power/toughness changes (+1/+1 or +0/+2), they are applied to Built Bear's printed power and toughness and are also part of Built Bear's copiable characteristics