Wikipedia has policies around what constitutes a reliable source, and academics who study a particular subject and publish in peer-reviewed journals are generally considered among the highest-quality sources. In this case, they nearly unanimously agree that what Israel has been doing in Gaza is a genocide.
It took Wikipedia a long time to come to this determination. At first, academics were divided, but as time went on and Israel's actions became ever more extreme, opinions shifted and nearly all academics in the field started calling it a genocide. That caused Wikipedia to start calling it a genocide. There was a very long process of discussion and debate on the talk page of the article leading up to this change, centered on an evaluation of the sources.
Jimmy Wales has now come along and essentially ordered the Wikipedia community to change the article. He's effectively ordering them to disregard the usual "reliable source" guidelines and instead represent a view that he personally feels is neutral.
The thing is that Wikipedia editors don't necessarily respect Jimmy Wales that much, and they generally don't think he has the right to dictate what any particular article should or should not say. Wikipedia has been around for more than 20 years. It has well established rules and a community of editors. Jimmy Wales is just the guy who originally set it up, but he's not necessarily an expert on anything.
