Its a format that has plenty of artifacts, and artifact decks can quickly go wide with all kinds of mana rocks and small permanents
Rile is good in Stuffy Doll decks, but also works in decks with Feather, the Redeemed as their Boros commander, as a cantrip that can target Feather
MaRo himself has recognized at multiple occasions that Planeswalkers had some of the smallest design space of all the card types (except maybe lands?) Due to how inherently powerful their concept is, providing mana and card advantage, and the fact they all have multiple abilities, and so many other restrictions, it is no wonder that many designs converge towards somewhat similar end results
Which brings us to our next explanation
While they have a great shuffle feel, just like regular Dragon Shield sleeves, the thickness is going to be an issue for a lot of people
Youll put a counter on the creature, and if it dies, you get to return it to the battlefield, usually getting more benefits when they enter