The main reason is mobile responsiveness. With <table>, you need to conform to all the table related elements like <tr>, <td>, etc, which is fine on desktop.
However when you want the table to collapse vertically instead of staying horizontal on mobile, trying to overwrite the browser defaults of these elements with CSS is a pain and not recommended. Using grid just makes it easier to change the display in CSS.
You could argue that using CSS display:table works too, but display:grid just brings a lot of other niceties for controlling how you want the page to look.