If you look at a rear shock which uses a metal coil, you'll always see a large visible coil sitting outside of the shock:
However, front suspension forks that use coil are visibly almost indistinguishable from forks with air springs because they hide the coil inside the stanchions. On the next picture the internal coil is partially visible; usually it is fully hidden inside:
Why don't we see forks with coils placed outside, the same way it always is for the rear shocks?
I can imagine that there may be some advantages of doing so, such as the coil protecting the shaft from rock strikes and scratches. Larger diameter coils may also open some design alternatives, such as thinner (lighter?) shafts. Clearly some disadvantages seems to outweigh them, but what are the disadvantages?