Upgrades aren't the only reason people buy computer parts. Replacement of failed parts is a huge market, and this fits in the market perfectly.
Socket 939 rigs were great, and still offer very acceptable performance today, especially for most home users that don't play games. The problem is that the parts tend to fail after so long, motherboards being a notorious failure point that is usually difficult to find a replacement for on EOL hardware, and even harder to find a cheap and good replacement.
The fact that s939 was the short period where AMD actually performed better than Intel, and many system integrators started switching to using AMD. There was a relative flood of s939 Pre-builts hitting the market. Pre-builts with weak motherboards, that tend to blow caps after 3-4 years of use... I still see one of two of these in my shop a week...
So this gives people the option to replace just the failed motherboard, and keep the good processor and RAM, instead of replacing the whole machine. People don't like replacing the entire machine if they don't have to, especially when the current machine still fills the need.
I don't know, I skipped 754 and went straight from Socket A to 939. Man, I still miss that 939 machine...Operon 148, 2GB DDR-500, x800GTO2...the glory days!!! Probably still my favorite build.