The reason, I think, is twofold
Expert - Pawmot and Pikachu ex The Expert deck can present a legitimate challenge: Pachirisu now has a function, adding Lightning Energy to Pikachu ex to allow it to attack more often while adding X Speed to retreat Pikachu ex after attacking
That one had an Ultimate box topper tucked into it, so it was extra special, and this pink collection will ensure your side of the battlefield pops
Until end of turn, you may look at the top card of the chosen player's library, you may play the top card of the chosen player's library, and you may spend mana as though it were mana of any color to cast that spell
That's still a lot of money, but it's a bit more budget-friendly than hunting cards from the base set
The consensus take on the Legacy subreddit seems to be, Eh, I might sideboard a few copies in certain matches, and I tend to agree