Why is a typewriter a better symbol (in The Glass Menagerie) than, say, a piano? After all, Laura could have been taking piano lessons.

I think that the typewriter and the typewriting classes
are a good symbol for Laura because it shows both her mother's attempts to normalize her
and her rejection of them.  Consider that Laura was enrolled in the typing classes by
her mother as a way of showcasing talents for "gentlemen callers." It seems unlikely
that Amanda was trying to develop marketable skills for Laura's entry into a work
setting, but I suppose one could surmise this, as well.  In any event, the typewriter
and its classes are Amanda's attempts to make Laura into something that is not
reflective of her own identity.  When she ditches the classes, she goes to the zoo.
 This world, the world of animals, and the world of her figurines represents her own
sense of self, and the embrace of it while rejecting her mother's attempts show Laura's
inner strength and her commitment to being her in a world that might not embrace it in
such depth.