Because the program it's used in is written in C too, as is most software that runs on Unix-like systems. It uses and depends on another C library (PCRE). The P2V system has to fit on a small bootable ISO, so minimising code size and dependencies is an overriding concern. Also it was written in 2014.

My thoughts exactly. I love using expect, but why would I write a C program when I quick and simple TCL script (expect is build with TCL and is threaded and easy to make thread-safe scripts).

There are also expect clones in Python and Perl (probably others, but I know and have used those both) if TCL is an issue for you.

This looks nice, but I don't see if as very useful.