Because it is possible with some really decent kit to read what was there before the previous overwrites occurred: hence the multiple passes.
This certainly applies to standard rotating hard drives
no idea if it possible with SSD's - ask GCHQ.
I'd take it outside and attack it a sledge hammer and smash the platters.
Which is probably one down from an ultra secure erasure which is where the magnetic material is sand blasted off the platters.