Why aren't there 8 double bonds in octasulfur (S8)?
An $\ce{S}$ atom in a molecule must have 2 bonds to be neutral and in the $\ce{S8}$ ring it needs both of these bonds to continue the chain/ring. If you introduce some double bonded S atoms into the ring each of these will have 3 bonds and will thus be positively charged. So, while reducing the number of free electron pairs you increase the charge of the compound which would be wrong because $\ce{S8}$ isn't charged. You can only use free electron pairs to form double bonds if you have some electron deficient atoms in the neighborhood which can "receive" them.