Network effects. John Ousterhout invented tcl in part to script/implement the Magic VLSI layout tool. So it's not surprising it's spread to other EDA applications.
that I couldn't tell you. I'd personally pick lua (or damn near anything else) first. maybe just entrenched knowledge/tooling in certain industries monster codebases.
Personally, I'd pick Tcl long before Lua... but I'd pick almost anything before Lua; which I find frustrating to work with. There's a _lot_ of personal preference and "feeling" that goes into what each person likes in a language.