Good question.

Aside from the difference in syntax (which is huge), I would say convenience.

SunSed = Hosting + IDE + CDN + a new programming language + Layout and Template system (like Jekyll and Hugo) + Many built-in tags to abstract things like embedding Youtube videos in a responsive way (which is not easy -- requires so many div wrappers).

You can also create your own tags and share it with others. I have started the process of creating tags for useful and popular libraries:

https://github.com/sunsed

If you know HTML, you already know the syntax of SunSed programming language, and can create fast dynamic websites with ease by abstracting your website content in custom HTML tags!

After all, SunSed is a programmable website builder.

I think when people are saying, “why not use X?” They mean, why not adopt X as your templating language system integrated into SunSed, instead of making your own.

Its called pug now, and you're right, it seems like a reimplementation of jade/pug