My 2018 Garmin Express Wish List is pretty short... a Linux version of Garmin Express so I could sync my Garmin Vivoactive HR without using Windows. Easy ... really.... there is already a Mac version.
A linux version of GE would be useful for a small subset of the user base.
Every new platform they support requires additional QA resources. Plus, there are multiple variations of Linux (Fedora, Debian, Slackware, etc) ... so each of those variations would have to be tested also.
Keep in mind, supporting MacOS doesn't mean a Linux version would be easy to implement ... Native Mac apps don't use the same UI & API functionality that Linux apps do.