https://www.gamespot.com/articles/hearthstones-new-legendary-spells-dont-work-the-wa/1100-6460700/

Hearthstone's next expansion, The Boomsday Project, is themed around arcane science--which lends itself to mechanical monstrosities and powerful spells alike. This will be the first expansion to introduce Legendary spells for each class, with powerful effects of their own. Lead designer Mike Donais shed some more light on how the team approached designing these new unique spells, and why they may not work exactly how the community expected.

In the lore, Dr. Boom has invited nine legendary (mad) scientists to his lab to run amok with their wild experiments, each with their own specialty. That makes up the nine Legendary class minions in this expansion, so many in the community have inferred that their signature Legendary spells would synergize with them for some powerful mega-combos. Donais said the team considered that, but ultimately decided against it.