The San Francisco Chronicle tries to explain why in a story today. Earlier stories by the Chronicle publicized accusations of cronyism by Teng, which contributed to her resignation. She's going through a divorce and says she's quitting for personal reasons.
Politics in San Francisco is crazy, and Asian Americans haven't put together the coalitions needed to be a real player in the city, and are often pitted against each other.
There's a huge population of Chinese Americans in San Francisco, many of whom are home owners and tend to be more conservative (which in San Francisco is a moderate Democrat). There are newer immigrants who don't take part in the process and various other factions from the left to the right. I think that's the misnomer, that there is a "Asian American" group that thinks of one mind. I suppose that's good thing for many reasons, but it doesn't help get people elected.