Because there is an incubation period, where people already infected start to develop first symptoms. There is a development period (from the first symptoms to

Because there is an incubation period, where people already infected start to develop first symptoms. There is a development period (from the first symptoms to the point where they are severe enough for the person to seek medical help — only at this point the case is registered). The introduction of quarantine measures is not immediately effective — there is a transition period of at least few days. There are response / case registration delays. And finally, it takes some time for the new epidemic regime just to be visible on the curve (a perceptive delay until the trend is understandable).