They were popular enough, and captured such perfect flavor, that Vehicles did something that doesn't happen often, it became immediately deciduous, almost evergreen
It provides immense flexibility because it hits multiple useful card types, and even if you mull down to 1 you're still seeing more cards than an opponent who has taken 0 mulligans
(If a creature with a shield counter would be dealt damage or destroyed, remove a shield counter from it instead.) Shield counters don't prevent players from sacrificing creatures
The Treasure token you get on turn 2 by beholding a dragon kicks things off quickly
The card synergizes with blink effects like Ephemerate and Teleportation Circle, letting you rack up counters faster
* Put a +1/+1 counter on each creature you control