A couple of reasons I guess. I was familiar with the Intel socket 370 chips and motherboards. I had plenty of SDRAM type memory and non-XP/non-P4 power supplies available (200-250 watts). And they are very cheap for the performance.
I just told someone else in the forum yesterday that I want to build myself a new XP or P4 system. It will probably have to wait until after Christmas however. Of course, I will have to get new memory, power supply, case, etc.