Because in English prefer simpler, shorter verb forms over longer, more complicated ones. That leading As I... (combined with the suddenly in the other clause) suffices to convey that the action in the first clause was a continuing one.
In languages that actually have both imperfect and perfect tenses, the first verb would be written in the imperfect inflection, the second in the perfect. But because English doesn’t have actual tenses (read: inflections) for that, we instead rely on other cues to convey those aspects.
Normally you use was plus an ‑ing verb to place special emphasis on the progressive or continuous aspect. But this isn’t cut and dry, and sometimes it would sound wrong to do it one way versus the other. Consider these sentences:
- Mary called while I was doing the dishes.
- Mary called when I was doing the dishes.
- Mary called when I did the dishes.
- Mary slept while I was doing the dishes.
- Mary was sleeping while I was doing the dishes.
- Mary slept while I did the dishes.
- Mary slept when I did the dishes.
- Mary was sleeping when I did the dishes.
Those are by no means all equivalent.