Short answers, pulled from the story.
The Game System License is a license released by Wizards of the Coast on the 17th of June, 2008, that allows third-party publishers to create products compatible with the intellectual property from D&D 4th edition. Unlike the older Open Game License, it grants access to a list of trademarks and phrases rather than the rules themselves, and it requires publishers to display a compatibility logo on their products.
Many publishers rejected the Game System License because of its restrictive terms, including a poison pill clause that prevented licensees from also publishing material compatible with 3rd edition under the older Open Game License. Wizards of the Coast also reserved the right to update the license at any time, binding all existing licensees to new terms, and required licensees to pay Wizards' legal costs in any litigation.
Clark Peterson, co-founder of Necromancer Games, declared the Game System License "an unmitigated disaster" after reviewing its terms and conditions. His company ceased its efforts to support the new edition as a result.
The Game System License's poison pill clause, which barred publishers from simultaneously supporting 3rd edition content, pushed many publishers toward Pathfinder, which Paizo built using the older Open Game License. Writing in 2012, Mordicai Knode noted that Pathfinder gave those who found 4th edition lacking a convenient rallying point during the edition wars.
The Game System License fell into disuse after Wizards of the Coast released the 5th edition of D&D in 2014 along with a System Reference Document for that edition under the Open Game License. It remains the only license available for 4th edition material.
The poison pill clause in the Game System License prevented any publisher using it from publishing material compatible with the older 3rd edition system under the Open Game License. This forced publishers to choose between the two editions and drove many to abandon 4th edition entirely. Wizards of the Coast eventually removed much of the most restrictive language, but the publishing community had already largely turned away.