In general, slow adoption rate of Windows 8 shows two things: the Windows 7 is good enough for the majority of consumers and that they do not want to transit to an operating system with unfamiliar interface.
Slow adoption of anything these days is simply due to not as many being purchased compared to years ago--
it is the economy-- where the daily life priorities of most folks outweigh even the advantages of Windows 8 and cause them not adopt it.
I realize many enthusiasts forget not everyone is as financially well off to dismiss the influence of the economy or competition, but to forget that regular folks don't share our passions (that not getting Win 8 must mean they don't like it either) is out-of-touch.