why delta region is suggested for BCG vaccination ?

Answer 1

deltoid? probably so its very visible and accessible to the one giving the vaccine

Answer 2

The BCG vaccine is injected intra-dermally - i.e. just under the upper layer of the skin - unlike most other vaccines which are injected deep into muscle.

The upper arm, near the insertion of the deltoid muscle is chosen to avoid the lymph nodes that can become inflamed if injected into by mistake. It also places the resultant scar where it can be easily checked in future to confirm that you are properly vaccinated, but is most often covered by a short sleeve.