"perfect replication is enemy of any robust system" - DAEMON by Daniel Suarez
This is the single biggest reason why windows systems/networks are captured so easily by worms, hackers and viruses. Each windows network is made of exact clones and if an attacker can pawn one system in the network he can annihilate each and every one of them. A typical corporate windows network is made up of identical systems running workstations, laptops, routers and servers. Once a box is compromised each and every one of them is compromised.
Compare this to a linux system/network. Even though it is possible to write a virus/worm for linux based systems, writing a virus which can affect every posix complaint system is at best improbable. Almost every *nix box is different from each other. Kernels, Desktop environments, distributions, servers, browsers etc all are be different and this makes is so much difficult to run an exploit on each of these machines in one go.