Lua is probably a bit of a special case, as it grew extremely popular due to its original niche (trivial integration in complex C and C++ projects): Lua was born in 1993, and started getting used in big-name games in the late 90s already (Baldur's Gate was scripted in Lua in 1998, so was Grim Fandango the same year). From there, it took over most of the gaming industry and could only leak out as a popular language.
from what i remember, tcl's popularity waned long before lua caught on in a big way. people were looking to ruby and python as extension languages, even though they had a far higher impedance mismatch than tcl, simply because they were vastly more popular as languages by then