The choice of which cards appear more frequently on the sheets appears more or less random (non-holo Flareon is more common than non-holo Jolteon and Vaporeon, for example)
These bloodsuckers have been mainstays in both casual and competitive play for years, giving players some of the most synergistic and aggressive gameplay loops in the game
Its a great card thats easy to build up while also punishing your opponent the more they build up
When the opponent is pegging one card to 2 life points (or whatever relationship, as "one card = Shock" is, in a sense, arbitrary), he or she is giving you a framework to exploit, at least when you can pick your cards
This effect was so strong that it is likely we will never see a direct reprint of it on any future card without any massive downsides
While Leyline of the Void is on the battlefield, nontoken creatures your opponents control won't die