Paper reference: 1
Becasue they are negatively charged and therefore stays within the lipid membrane. This would not be the case with neutral molecules, which might dissolve and diffuse into the outside.
Requires a connecting link that is atleast divalent.
Sulphur and Arsenic are too reactive and toxic as well.
Phosphates are kinetically stable at neutral pH, but can be activated by enzymes.
Linking unit should have a third ionizable group in addition to two divalent groups.