How did Trulia came out of nowhere to steal the thunder from Zillow?

The first version of Trulia was bad. Jackass bad. It's like those crappy Google Maps mashup site built by a kid overnight. When Zillow came out, I thought they had won the real estate web sector. That's it. Then suddenly, out of nowhere came a revamped Trulia with a better site than the first version and most importantly, MORE HOUSE SALE LISTINGS than Zillow. What's a real estate site without real estate sale listings?