While the artwork definitely has something to do with its popularity, its ability doesn't hurt either
Jafar, Striking Illusionist is an Amethyst Ink, Floodborn card with much stronger stats than Ursula at first glance
You can't attack or block with a Planeswalker, but they're also not affected by summoning sickness and can use their Loyalty abilities as soon as they enter the battlefield
By that I mean Verdurous Gearhulk will be an 8/8 when you need an 8/8, it will make two giant bodies to get past blockers when your opponent has blockers, and it will spread counters on your team when you need to go wide
Torch of Defiance on average returns about four mana in two turns with the mana and the -3 but she can get way more if she manages to hit the right thing
Collectors also invest huge sums in out-of-print cards and those featuring popular female characters