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Nothing in life is free. Decreasing the footprint (area requirement) of most water treatment processes (hence increasing the rate) increases the energy requirement, complexity and potential for things to go wrong.

Another thing that you miss when going from slow sand to rapid sand filters is the schmutzdecke - a biological surface layer that you get with slow sand filtration. This can take out many contaminants that rapid sand filters can't.

That said I wouldn't quite agree that there is no contest. The massive benefit of rapid sand filters is the reduction in footprint per m3 of water treated. In high population density areas this often makes rapid sand filtration the only conventional choice.