The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
$3 and a Buy for only $3 is a lot of money, despite the downside of Exiling on-play. Being able to open with any other $4 card and feel pretty good about hitting $5 or even $6 makes it difficult to not get at least one Stockpile in the opening, and it’s fairly easy to buy one additional Stockpile right before each shuffle to keep them in your deck.
Stockpile is a card that is much more centralizing in 2-player games, because uncontested access to the pile lets you have amazing turns throughout the game, so you can be locked into contesting them until the pile runs out. With more players, the early-game benefits are still pronounced, but contesting the pile is less of a concern.