I've talked and written recently about why "just pick[ing] up Lua or Python" was not an option, but there are other strong reasons that was not in the cards.
Think about what mid-1990s Lua and Python were like, how much they needed to change in incompatible ways. The web browsers would never have tolerated that -- you'd get a fly-in-amber-from-1995-or-1996 version of Python or Lua, forced into a standards body such as Ecma (then ECMA), and then evolved slowly _a la_ ECMA-262 editions 2, 3, and (much later, after the big ES4 fight) edition 5.
Interoperation is hard, extant C Python and Lua runtimes were OS-dependent and as full of security holes, if not more full, than JS in early browsers, and yet these languages and others such as Perl were also destined to evolve rapidly in their virtuously-cycling open source communities, including server-side Linux and the BSDs (also games, especially in Lua's case -- Python too, but note the forking if not fly-in-amber effects: Stackless Python in Eve Online, e.g.).
JS, in contrast, after stagnation, has emerged with new, often rapidly interoperating, de-facto (getters, setters, array extras, JSON) and now de-jure (ES5) standards, the latter a detailed spec that far surpasses C and Scheme, say, in level of detail (for interop -- C and Scheme favor optimizing compiler writers and underspecify on purpose, e.g. order of evaluation).
The other languages you cite have been defined normatively over most or all of their evolving lives entirely by what their C implementations do. Code as spec, single source implementations do not cut it on the web, what with multiple competing open- and closed-source browsers.
Think about what mid-1990s Lua and Python were like, how much they needed to change in incompatible ways. The web browsers would never have tolerated that -- you'd get a fly-in-amber-from-1995-or-1996 version of Python or Lua, forced into a standards body such as Ecma (then ECMA), and then evolved slowly _a la_ ECMA-262 editions 2, 3, and (much later, after the big ES4 fight) edition 5.
Interoperation is hard, extant C Python and Lua runtimes were OS-dependent and as full of security holes, if not more full, than JS in early browsers, and yet these languages and others such as Perl were also destined to evolve rapidly in their virtuously-cycling open source communities, including server-side Linux and the BSDs (also games, especially in Lua's case -- Python too, but note the forking if not fly-in-amber effects: Stackless Python in Eve Online, e.g.).
JS, in contrast, after stagnation, has emerged with new, often rapidly interoperating, de-facto (getters, setters, array extras, JSON) and now de-jure (ES5) standards, the latter a detailed spec that far surpasses C and Scheme, say, in level of detail (for interop -- C and Scheme favor optimizing compiler writers and underspecify on purpose, e.g. order of evaluation).
The other languages you cite have been defined normatively over most or all of their evolving lives entirely by what their C implementations do. Code as spec, single source implementations do not cut it on the web, what with multiple competing open- and closed-source browsers.