[herge]

To quote Donald Knuth, a 64 bit program would "waste half the memory [and] effectively throw away half of the cache".

You may double the number of registers, but you are still shuffling tons of data between your caches and memory, and that is going to be your constraint for most programs. Losing half of the 4mb or 12mb of your L2 or L3 cache because you use 64 bit pointers has a much bigger impact than doubling the number of registers.