BB, as her uniquely strong card-locking gimmick is, well, unique. While only very rarely optimal (particularly when you already have the buffs and just want to NPXX), the ability to slam the enemy with say an NPQQ from Kintoki (Rider) (or any other DPS with strong NP uptime), is still passably effective at least.
XX by comparison is an odd duck in an odd class container with an also odd give-and-take, trade-off-filled kit (e.g. 1T invul + 1T ATK Up). They’re both luxury options fundamentally, but literally no one else specifically has BB’s ability to card-lock and the strength that can entail. (For the purposes of buff-stacking and damage racing it’s actually quite easy to do without even if you’re not trying to do a TA in general, though I phrased it in that very particular way for a reason.)
XX is always hitting neutral (x1.0 no less), is Arts meaning she’s slower/harder to enable than e.g. Merlin Smash or Skadi Slaughter if you’re after Big Numbers , and altogether is a very replaceable unit by virtue of her being meant to be so splashable without much else going for her.