Circle CI has been steadily taking marketshare away from Travis for many, many years. And in my personal experience, mostly because of Travis' slow pace of improvement. Using containers for testing blows away performance on Travis for comparable tasks. Travis had made some improvements this last year to their workflows, configuration, and platform, but too little too late. My experience in dealing with their customer service (we did have a paid plan for some time) and customer feedback (feature requests, pleas for fixes, etc) was also quite poor.
I'd moved to Circle CI two years ago, and the only tasks/projects of mine still running on Travis are those which are deprecated, in suspended animation, or abandoned. For myself and my immediate peers, Travis is obsolete, and they did it to themselves. With Azure pipelines now a thing (and also far superior to Travis) I see another slow, slow death of a pioneering service.