I believe that we can glean a hint of the answer in Mark Rosewater's latest State of Design article, from July this year
Keldeo EX (EX) is the perfect attacker, since it does more damage for each Water Energy attached to it
Four mana for a counterspell is a lot, but if you can work it in on an early turn, it allows you to build your board while holding up more answers, which is a point these decks love to get to
And They All Lived Happily Ever After And that wraps up my card-by-card design stories of Throne of Eldraine
Brainstorm Another card that draws three and only costs one, Brainstorm is a strictly worse version of our number one card, but its still powerful enough to take the number three spot
I cannot wait to unearth more of these amazing connections that bridge the realms of imagination and reality