Apache is allocating each "slow loris" a process (or at least a thread) in the expectation that it will soon get a complete request, process it, and return the thread / process to the pool. Apache can afford to allow only a few of these to exist at one time (limited by your hardware and/or chosen configuration)
In contrast the design in products like IIS allocates nothing but a socket and a little bit of working space until the "slow loris" has written out a whole request which can be answered. Many, many more such requests can be tolerated on the same hardware.
Thus, a thousand "slow loris" connections to your IIS are merely an annoyance, nothing that needs urgent action, while on Apache they've probably effectively taken your site off the Internet.