These bots you are talking about are not intelligent, they do not find "insecure" servers to break into. They simply brute-force and exploit known bugs on popular services.
There is no botnet targeting web services running on DOS, because no one is running web services on DOS.
Finding insecure servers, what human hackers would do, requires persistence, time and a working brain.
Bots, instead, throw shit at a wall and see what sticks. Move your SSH server with credentials root:root on port 1234 and notice how many bots get utterly defeated (only for sake of argument, because OpenSSH has a banner which makes it easy to identify wherever it's running)
There is no botnet targeting web services running on DOS, because no one is running web services on DOS.