I think this is because they’ve had such a hard time balancing standard, that they’re afraid to put too much power into any set. There was a solid period of having a top tier deck emerging, and then being banned, then re-emerging, and being even more banned... I can’t find the quote, but I’m sure I remember Rosewater saying something to the effect of they can’t put fetches into standard because they’d be too powerful, for example.