It was developed by ESRI. On that page, in the FAQ, they state:
You may find yourself asking, "Why did Esri bother writing it's own language when it probably could have used others, like Python?"
This decision was made because we needed an expression language that met the following requirements:
- Lightweight and simple - A language with a small number of functions and an easy syntax
- Portable - A language that can execute in multiple environments with the same syntax for web, desktop, and mobile apps
- Secure - A language that doesn't have security vulnerabilities that open the door to malicious scripts
- Geospatial first - A language that makes geospatial functions and capabilities first-class citizens
Developing our own scripting language was the best option because other alternatives, such as Python and JavaScript, couldn't check all of the boxes above either due to language size, or their propensities for security issues.
Here's the original introduction blog post: Introducing Arcade , however it doesn't go any further in depth with your questions.
