Fundamental issue is that programs use system calls to communicate with the outside world, and most of those system calls deal (sometimes indirectly) with files. For a network filesystem client going through the kernel, then to userspace and back again is just a stupid way of doing something relatively simple.
To sum up, network filesystem clients are in-kernel for all the same reasons why normal filesystems are in-kernel. For network fs servers it's a slightly different trade-off.
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