In the 1990s, an awful lot of Linux apps were Tk/Tcl. People complained then about them not being "native", but they were slimmer and better performing than the typical Java or Electron app of today.
It was better looking than Athena or OpenLook, which were your other two free options back then. GTK+ wasn't around, QT was mostly unknown and had license issues anyway, and Motif cost money.