Why is it that bacterial ribosomes can begin translation before mrna synthesis is completed, but eukaryotic ribosomes cannot? see section 17.3 (page 356) ?

This happens because in prokaryotes (including bacteria) both translation and transcription occur in the cytoplasm while in eukaryotes those processes are separated in space. Transcription and translation in prokaryotes are continuous and simultaneous processes. Transcription in eukaryotes is separated from translation. Transcription occurs in the nucleus where the formed transcript proceeds and then transported to the cytoplasm where translation occurs.