I think it is mostly for historical reasons. When Intel made the 4040 processor they made the decision to use Von Neuman, possibly becuase at the time it seemed the sensible thing to do. Each generation adopted the same system to be as close as possible to software compatible. So the 4004 became in turn the 4040, 8080, 8086, 80186, 80286, 80386, 80486 then Pentium (80586 was dropped for legal reasons). Intel and other companies made 'spin off' products such as the Z80 series so they could be used as 'up market' replacements for some of these but still share the instruction set.
With the PC architecture being adopted so widely, it wouldn't make sense to suddenly change to a different bus system as it would mean a complete new market to compete in.
Interestingly, back about 15 years ago, Centaur made a drop-in replacement for the Pentium which was actually a Harvard RISC processor emulating a real Pentium. It was faster than the Pentium at the time but couldn't keep pace with Intel for long.
Brian.