it is very likely due to the influence of warm weather, which suggests that with the arrival of winter the virus may become much more active again.
warm weather may help a tiny bit but there is no evidence of it being able to reduce the effects of this virus yet. the only reason why the situation is improving in any area is first and foremost because people are more aware now and they are following the basic guidelines to stop the spread of it (wearing masks, social distancing,...).