The reason is
You can manage DOM directly in simpler projects or traditional server-side MVC projects but for large-scale projects wise decision is to maintain your code using component and for faster load and performance use virtual DOM.
For Ajax, newer, better API like fetch() is replacing jQuery, so it’s meaning that all strength of jQuery is no more.
Also, another reason is the single page Application,
What is the single page application
A single-page application is a web application or website that interacts with the user by dynamically rewriting the current page rather than loading entire new pages from a server.
You can easily make a single page web application using a modern framework and library like React, Angular. But It’s hard in jQuery.