There are two primary issues. First is the performance of a monolithic kernel versus a message-passing microkernel. Linux has been tuned pretty extensively; as

There are two primary issues. First is the performance of a monolithic kernel versus a message-passing microkernel. Linux has been tuned pretty extensively; as a monolithic kernel with a lot of eyeballs trained on it, it's hard to beat. Second is the fact that the vast majority of OS kernel source represents deep OS _design decisions_. There are a lot of decisions to make. If you're going to make a lot of decisions that are the same as the Linux set, there's not much point in writing your own code. User space has a set of expectations; Linux conforms. Traction is difficult with non-conformance.