"Unfortunately, as Windows 10 has grown in adoption and usage, we have seen some software programs circumvent the design of Windows 10 and redirect you to search providers that were not designed to work with Cortana," GM of search and Cortana Ryan Gavin explained in a blog post. "The continuity of these types of task completion scenarios is disrupted if Cortana can't depend on Bing as the search provider and Microsoft Edge as the browser." In other words, using other apps led to a less reliable experience.
This doesn't mean you can't continue to use Google search and Chrome as your primary browser on your Windows 10 machine. You can certainly still do that, and set those and other third-party apps as the default option to hop around the internet. However, you'll want to keep in mind that Cortana will only provide answers to questions and lend a hand with tasks through Microsoft Edge and Bing.