I'm curious where you saw it doesn't spread as fast as the flu. From what I've read, typical flu has an R nought of around 1.25-1.5, with a severe pandemic like 1918 closer to 2. I've seen estimates for COVID19 ranging from 2 to 6. Case tracking and diligent isolation help lower that transmission rate, but once community spread takes hold even measures like China took can only reduce the infection rate so much. It does buy time though, which is important in preventing healthcare systems from breaking down. The trade off is, to prevent unchecked growth, you have to basically shut down your economy, which has its own knock on risks.