Because they made a calculation that the assistant product has broader appeal. Including either product has the potential to acquire users, and retain other users, but they think their target market has a lot more people who might enjoy using the assistant, than they do people who want a search API as a free addon. Or that someone who starts using the assistant is more valuable (due to stronger retention) than someone who uses the search API because the latter is more of a commodity that you can replace in 5 minutes if a new startup offers a cheaper option.