why does it come as " I was" and not " I were"?

Dear Student, 

'Was' is used with the singular pronoun 'I'. 

Eg : I was tired.

'Were' is used with the plural pronoun 'we'.

Eg : We were tired.

However, for the subjunctive mood, we use 'I were'. A verb is in the subjunctive mood when it expresses a condition that is doubtful or not factual.

Eg : I wish I were you.

Regards,