Welcome mAsTa NoOdLe to Xp-erience.org.
Well I not to sure what to think bout your question as I'm not sure what you mean by 64-bit processors. If you are talking about a processor that has a 64-bit address bus then god only knows why the its taken this long. If you are talking about the bus inside the cpu itself I know the Pentium 4 as 64-bit buses on the processor itself and I would guess that the Athlon's have the same idea.
The reason we haven't seen any (affordable) 64-bit CPU:s is that 64 bits will require double the amount of "legs" on the CPU. Ridiculus as it might seem, this is the most expenisive part of the manufacturing, legs and casing. You also get problems with the length of the wires from the silicon chip to the outside of the chip. At clockspeeds nearing 3 GHz a signal can only travel about 6 cm every clock tick. Distributing all the signals over the motherboard will also be at least double as hard (longer wires as a result here too). And 128 bit you ask. What a nightmare. We will probably get there as well some day, but it'll take a while. Are you happy with the answer?
64 bit looks like fun if you ask me, at least from a developing point of view, but it also means forking out for a 64bit OS, and buying 64 bit apps. 16 bit apps will be blow out of the water (NO MORE COMMANDER KEEN! NOOO!), and 32's shoud be able to be emulated I believe.
However I think we should work on lowering the price of multi-CPU systems first. I'd rather have 2 Athlon MP's than a 64 bit processor.