It's hard to claim that something developed for a different OS is derivative of the Linux kernel.
There are also cases like AFS where the filesystem existed before Linux, in those cases it's hard to argue that something that existed before Linux is derivative of the Linux kernel.
For the other cases, it boils down to nobody taking the time to go through all the legal hassles of pursuing the lawsuit. It's quite a hassle, and in most cases there is a rather limited benefit (see Rob Landley's comments on the lack of benefit from the busybox lawsuits and his reasons for withdrawing from GPL compliance efforts) It takes really annoying someone to trigger action.
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