We needed a language that matched all of the following criteria:
* Freely available
* Embeddable integrate within pdfTeX
* Very small footprint
* Portable
* Easy to extend with pdfTeX-specific functionality
* Fun to work with
Lua was the first language to match all the criteria. The known scripting languages tend to be much too large for our use. Specifically, we have rejected Java, Perl, Python, Ruby, Scheme on one or more of those criteria.