It's a consequence of javascript being "good enough." Originally, the goal was for the web to support multiple languages (I think one prototype of the <script> tag had a "type=text/tcl") and IE supported VBScript for a while.
But at the end of the day, you only really need one, and the type attribute was phased out of the script tag entirely, and Javascript won.
But at the end of the day, you only really need one, and the type attribute was phased out of the script tag entirely, and Javascript won.