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Some of them probably aren't worth including. Cards that're identical or strictly worse than others, cards that were only played for budget reasons, or cards the player just thought would be funny to have in their side.
They were all cards that, basically, one or two players had in their sideboard. Many of them were cards that showed up literally once across all the decklists.
Nothing was left out for complexity or any reasons like that. They were just the least-popular cards that still see a tiny bit of play. And, if any of them prove important, we'll add them in down the road.
There are so many reasons why this could be. Maybe some of them already exist as reprints with different names. Maybe some of them were just very inferior cards.
The most obvious reason though is that Pioneer Masters is a draft set and there is a limit on how many cards you can just sneak in for constructed reasons.
This is exactly right. We made sure to include the key cards, that saw real play. Cards on the fringes of play, like these, got included or excluded based on building a better draft environment