Everyone else mentioned the app support which is true, but for graphene specifically, they do not like desktop Linux at all because they don't like its security. They would much rather build on AOSP than desktop linux.
True, security seems to be the main reason. But while desktop linux is community driven, AOSP is controlled by Google. So long-term, GrapheneOS is at the mercy of Google, no matter what. And also in terms of security, Google is becoming more hostile, where since a while, the GrapheneOS team is not allowed to publish all security patches source code in a timely manner: https://discuss.grapheneos.org/d/26476-backports-quarterly-r...
True, security seems to be the main reason. But while desktop linux is community driven, AOSP is controlled by Google. So long-term, GrapheneOS is at the mercy of Google, no matter what. And also in terms of security, Google is becoming more hostile, where since a while, the GrapheneOS team is not allowed to publish all security patches source code in a timely manner: https://discuss.grapheneos.org/d/26476-backports-quarterly-r...