Like most Pokmon cards, the value of Pikachu ex depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Prismatic Evolutions)
Its body is covered in glowing crystals that release toxic spores to defend itself
For these decks, the opportunity of playing Jegantha, the Wellspring is just too high, not because of the number of cards they can't play, but because of the power-level of the small number of cards that don't meet Jegantha, the Wellspring companion restriction
The last part of that answer is interesting
This Charizard ex features a stunning illustration by Mitsuhiro Arita that connects with the Charmander AR and Charmeleon AR cards to form a three-card story sequence a design concept introduced in the Scarlet & Violet series