[f0r]

Because the Tcl community is 10+ years behind everyone, and that's still their goto code hosting site.

[monetus]

http://chiselapp.com/ is where most of the active TCL repositories are stored, at least for fossil. For git, it seems to be a mix of GitHub and gitlab.

[grall]

The main repositories for Tcl, Tk, and a few near-core packages are at https://core.tcl-lang.org/index.html

Some of these are also mirrored over to https://github.com/tcltk/