The reason why it doesn't use the Chrome DevTools Protocol is that it (Helium) is a wrapper around Selenium WebDriver [0].
One advantage of wrapping WebDriver, rather than using something like the Chrome DevTools Protocol, is that WebDriver has an interface specified by an W3C Standard [1] [2], and can be implemented for any browser. The Chrome DevTools Protocol (obviously) only works with browsers based on Chrome.
Apparently the reason one would keep using selenium over headless chrome (perhaps using a wrapper like puppeteer [1]) is that selenium works over many different browsers, not just chrome.
"WebDriver can be used with all major browsers. Automate real user interactions in Firefox, Safari, Edge, Chrome, Internet Explorer and more!" [2]
One advantage of wrapping WebDriver, rather than using something like the Chrome DevTools Protocol, is that WebDriver has an interface specified by an W3C Standard [1] [2], and can be implemented for any browser. The Chrome DevTools Protocol (obviously) only works with browsers based on Chrome.
[0] https://www.selenium.dev/documentation/en/webdriver/
[1] https://www.w3.org/TR/2018/REC-webdriver1-20180605/
[2] https://github.com/w3c/webdriver