Leaven is an "agent of influence." Just focusing on the "symbol" of leaven in any passage is to run the risk of missing the point of that passage. That's what's wrong with the leaven=sin/evil concept. In Matthew 13, the Kingdom of God itself, and by extension Kingdom citizenry, is identified as a agent of influence in the world. The Passover associations from Exodus, and picked up by Paul in 1 Corinthians, were meant to symbolize the radical nature of deliverance from the old life. The Israelites were to get rid of all the "influence" of Egypt, not carrying any of it with them as they departed, as they were saved out of their bondage. The new life of freedom was to have its own untainted influence on them. The Israelites were not expected to live off unleavened bread perpetually. New leaven would be procured. Post-passover leaven was reintroduced to the house not as sin, but as new influences.
The Passover was repeated annually. It represented on an ongoing basis the physical salvation of the nation, which in turn represented the necessity of spiritual salvtion. It also represented the ongoing responsibility of the people to self-examination. Once a year at Passover time (kinda like new-year's resolutions) Israel purged out the old leaven. They conducted (or should have) a spiritual inventory. They were to purge out all the "Egyptian influences" in their lives. Then they would be like a "new lump." The traditional bread-making and leavening process is actually one, long baking event, lasting sometimes for years. A bit of the dough is saved after every baking and this "starter" is kneaded into the next time to the new batch of dough. The new batch really is the old batch.
But a "new lump" has no starter in it. It is unleavened. The maid is hungry. She makes some dough. But she has no yeast for it, no starter. Unless she delays, starving, until she purchases some yeast, or (more likely) until she has grown some in a culture, she must make her first loaves as unleavened. Such is the Paschal symbolism, for Christ our Passover Lamb has been sacrificed.
