According to the documentation provided by three.js:
Due to limitations in the ANGLE layer, when using the WebGL renderer on Windows platforms, linewidth will always be set to 1 regardless of the specified value.
Wide lines may not work in certain browsers that do not utilize ANGLE, such as IE11
https://i.sstatic.net/t64In.png
This question discusses potential workarounds for this issue.
Is there a way to determine if the browser supports wide lines?