The switch to Hongmeng would follow Google's decision to prevent Huawei from using certain aspects of Android--such as the Google Play Store and Google's applications--after the company was blacklisted by the U.S. Department of Commerce in May. Huawei could technically still use Android because it's partly open source, but many Android users are probably accustomed to having easy access to Google's services on the platform.
Chinese users don't use Google's services, on Android (Google services are banned by the Chinese government). So, for phones targeted for sale inside China, there has to be another reason. I think it's more likely all the binary drivers and proprietary firmware in a modern smartphone.