Why does solid phase peptide synthesis (SPPS) always proceed from the C to the N terminus?
The reasons are 


The use of an acylating agent in excess (e. g. acetic anhydride or N-acetyl imidazole) drives the reaction forward.
UNCAs (urethane-type protected  N-carboxy anhydrides (e. g. Fmoc, Z, Boc, ...)) can be used. They prevent racemization of the amino acid, that it is attached to. 


But there are (despite of ribosomes themselves) also instances, where N -> C directionality was applied:


http://onlinelibrary.wiley.com/doi/10.1002/anie.199710991/abstract
http://www.ncbi.nlm.nih.gov/pubmed/10891165