When it gets crushed by the passing bolts of magnets which consume the entire tube diameter because they are propelled by air and need to fill the entire tube in order to work.

If you want to run fibre along, and it's a good idea while you're digging away, do it in a parallel but separate tube.

I think that was the idea of the parent comment, just put anything else in the hole in the ground. Fiber is welcome :)