You didn't need to have bought Vista. Some of us stayed with previous OP such as XP. Windows XP, or Windows NT 5.1, is said to be stable due to the fact it was "tested" through Windows 2000, or Windows NT 5.0. Windows 7 being actually Windows NT 6.1 with Windows Vista being Windows NT 6.0, some people know the rule have skipped Vista.
Also, I personally thought word of mouth information about Vista didn't sound that tempting except some security features to my ears.
After the final release of Vista, when people really started using it, the sparkle of the marketing promotion kind of faded. The thing is that unless you were the kind of person that custom built your computer, you had little choice but to stick to what you had (ie XP or whatever... my case win 95
) or buy a new machine (with vista pre-installed).
The majority of users will avoid custom building machines because they are not experienced (my self included...
) so little choice but to go with the flow.