it's absolutely the kitchen sink standard library and "complex" language that makes it worth using, because 1) it means no additional dependencies and 2) the language is expressive enough to let the template syntax remain simple and well-abstracted
have you ever tried bootstrapping python? because it's really not a big deal (the only hard dependencies are a C compiler, libffi, and zlib, though distros also add libedit or readline, expat, sqlite, bzip2, xz, and openssl, all trivial dependencies present in more or less any system); tkinter is pretty much always compiled and packaged separately
meanwhile with lua you'd need many additional modules, and the templates would be verbose and far less readable
a non-exhaustive list of stuff python provides that would need to be provided separately:
1) subprocess management
2) path parsing (and normalization, manipulation, etc.)
3) various filesystem operations
4) shell command lexing
5) globs and fnmatch patterns
6) json
7) date/time handling
8) filesystem advisory locks
9) temporary file handling
10) http client (for sources fetching without external subprocess)
11) termios and ptys (for isolation of controlling terminal)
12) regex
13) hashing
14) topological graph sorting
15) cmdline argument parsing
16) readline functionality
17) thread/task pools
and so on and so on
