Why did Siri's co-founder leave Apple when Google's AI chief took over?

Apple’s AI assistant fell behind while Tom Gruber was the head of Siri Advanced Development.

From the Information ($):

Tom Gruber, one of the original Siri cofounders, has left Apple, where he was recently the head of Siri’s Advanced Development group, according to multiple people familiar with the matter. Apple’s head of search, Vipul Ved Prakash, has also left the company… His departure comes as the Siri group is undergoing a major leadership change, with the announcement last week that John Giannandrea, the former Google artificial intelligence research and search chief who joined Apple in April, would take over the unit…

The Siri unit has [seen] considerable leadership turnover in recent years. Mr. Giannandrea took over the group from Craig Federighi, Apple’s senior vice president of software engineering, who had assumed leadership of the unit in 2017. Before that Eddy Cue, Apple’s senior vice president of Internet software and services, oversaw Siri.

Underneath these executives, Siri is run by Bill Stasior, a search expert from Amazon that former Apple executive Scott Forstall recruited before he was fired in 2012. In addition to Siri, Mr. Stasior is tasked with leading Apple’s search team.

My take: Heads finally got cracked. If Giannandrea delivers, we’ll know whom to blame.

See also: The Information: Why Siri sucks.