Because all of Intel's Pentium 4's, even with hyperthreading, have been completely obliterated by AMDs new 64 bit chips in nearly all benchmarks, it's a cold hard fact and you can't ignore it.
Also, there aren't any operating systems out at the moment which are based on 64 bit architechture, so I don't know where you are pulling that from. However, since the release of the AMD 64 bit chips, companies like Red Hat, Windows and others have begun developing programs that DO run on 64 bit architechture.
It's silly to build software that can't even run, that is the why the 64 bit proccesors have come first, so that software to take advantage of them can actually be used. Basically AMD has given these companies a little push in the right direction, and eventually 64 bit proccesors will replace 32 bit, so why, when you know this, would you purchase a 32 bit proccesor?
It's the same as the newer 16x DVD burners. 16x media is incredibly difficult to find, so why buy a burner that can burn at those speeds?
Whether 64 bit applications are availible now or not, the AMD 64 is still a far superior 32 bit proccesor when compared to the intel pentium 4.