After the introduction of the latest patch for Heroes of the Storm, Blizzard has decided that the Hero League needed further changing. On the game's official website the studio has revealed the reasoning behind the decision, claiming parties of three and four players often result in "large skill gaps among party members."
The developer pointed out that "large parties represent a a small enough portion of Ranked games that we're comfortable removing them from Hero League in order to improve overall match quality." The new solo and duo restrictions on Hero League will be implemented in the next game patch.
In the meantime, you can meet the newest hero added to the roster - Rexxar - right here