Here's why:
* Am is the first person singular present tense form of the verb "to be".
* Was is the past tense form of "to be" for all singular subjects (I, he, she, it).
* Was is also the past participle of "to be" for all subjects.
Examples:
* Present tense: I am happy.
* Past tense: I was happy.
* Past participle: I have been happy.