Why OnShape defines a new program language - FeatureScript?

I get asked this every now and then -- it was not a light decision.  The advantages of using an existing language is an easier learning curve like you mention (for those that know the language), and the ability to reuse some of the existing language parsing/engine.  The main reasons we decided to develop our language are:

1. Determinism -- regeneration has to happen exactly the same way every time.  So we'd have to strip down any existing language to remove external interfaces, randomness, date/time functionality, as well as subtler things like unsorted map iteration and uncontrolled side effects. (See also https://cad.onshape.com/FsDoc/intro.html)
2. Control -- the ability to make decisions regarding the language (e.g., the design of the importing/module/versioning system, error handling) has proven very valuable, as we didn't know all the issues and design decisions we'd run into at the start.
3. Operator overloading -- for working with units and vectors (which are essential to CAD) to have readable syntax, you have to overload operators, which JS does not support.  Python and Lua (other languages we considered) do, but Python is more complex than we'd like and relatively few people are familiar with Lua, negating one of the primary benefits.
4. Rollback -- we have to be able to undo portions of the execution efficiently, which makes adapting an existing execution engine very difficult.

What we have found is that it takes most people a couple of full days of working with FS to become familiar with syntax and semantics.  After that, writing FS gets much easier and remaining issues are typically around geometry and library familiarity (which wouldn't be any easier in another language).