Most likely answer is "because that's what they wanted to do", which is a totally legit reason.
If the goal is to run scheme, run scheme. If the goal is to port Hoop then I get it.
Why walk in water when you can take the ferry?
- For the same reasons people run js server-side: single codebase in the same language; familiarity with the language; other?
- Because some people simply like scheme and want to code the frontend of webapps in scheme
- Because they want to port Goblins to the browser and Goblins already work with scheme.
Most likely answer is "because that's what they wanted to do", which is a totally legit reason.