A fundamental law of nature is the driver behind this. There is no grass in the forest, there can be several bacteria in one big petri-dish but never in the same spot, ... . The same types of viruses compete for the same resources.
Update April 2023: Peek-a-Boo Flu
Martin Neil, Jonathan Engler, Prof Norman Fenton, and Nick Hudson
investigated this problem and wrote a substack article about it: