Why not use an ordinary aluminum pipe? :O Why is it even glass? Mine is aluminum. Isnt it supposed to be aluminum? :O
1) Aluminum bends and stays bent. The original (1970's) tiller extensions were 1/2" Al extrusions that ended up looking like noodles after a short while. If you dropped the extension and it got swept back under the tiller, it would end up with a 90 degree bend in it - if it didn't break first.
2) Fiberglass extensions were introduced in the late '70s / early '80s as one of the first electrocution prevention / safety measures developed by Hobie. There was a huge extension exchange program and thousands of free fiberglass extensions were distributed.
Until recently, the class rules for all Hobies except the Tiger and Wild Cat required a "non-conductive" tiller extension - which precluded carbon fiber / epoxy extensions (very lightweight and stiff). Sailors argued that "non-conductive" was too ambiguous - anything will conduct electricity if you put enough voltage across it (even fiberglass - especially if it's damp with salt water). So the rule was changed to allow any extension material, with a strong recommendation that they be non-conductive.