This plugin makes use of the core WordPress customizer which is a built-in functionality in WordPress. Creating a back-end panel actually requires loading extra functions and code so it’s not as optimized. By making use of functionality alredy available in WordPress we make use of the “DRY approach” when it comes to coding to keep things as slim and fast as possible. With the added benefit of allowing you to preview the changes live. Plus, as long as you are using a quality host the Customizer should load just as quickly as a backend admin panel.
I would love to know why you find a backend options panel easier to use, perhaps I have some suggestions. This may be something you’d like to bring up to WordPress.org though because the Customizer UI is something built-into WordPress and not the theme itself. I’m sure they’d love the feedback – https://core.trac.wordpress.org/
For example, if you are a developer and rather not have access to the Customizer settings you can set all the options using code in your child theme functions.php file. This way you don’t even need a UI 😉 I can help show you how to do this if you’d like.