I think the 'failure' of the commercial Linux desktop can mostly be attributed to economics. Most importantly, consumers do not want to pay for a desktop operating system (even if it were proprietary). Microsoft effectively shut down the alternative market by making Windows-hardware coupling mandatory for large hardware vendors. This gives the perception (through scale and coupling) that Windows is nearly free. So, why would one spend money on an alternative operating system?

Then, there are social factors. A vocal minority of the Linux community is outright hostile towards proprietary software or even towards any form of capitalism. It is not a very attractive market to operate in - one misstep, and elaborate PR campaigns are set up.