I think there is another reason, besides Magic being scripted in Tcl. Back in the days when some EDA companies decided on Tcl there were only a few languages which could be embedded. Tcl was the only one with some popularity, documentation about embedding including sample code and a commercially viable license. For Perl for instance the embedding feature was largely undocumented at the time (early 90s I think) and Perl is GPL which is a no-go for commercial software. Ruby did not exist yet and Python maybe just was not visible enough.