The cost of currency and relative lack of availability of instrument airports would certainly be a factor. Another consideration, I would imagine (with no statistical evidence to prove this so please feel free to correct me anyone) that there are more pilots in the USA who use their PPLs for commuting, whether it be for work or leisure. An instrument rating, while it is indisputably going to produce a more well rounded and able pilot, is difficult to justify for the average PPL who wants to take their family and friends up on nice smooth days to get lunch somewhere close by or check out the local scenery. Poking around in low cloud with the likelihood of turbulence, icing and all that sort of thing probably doesn't appeal much to someone flying purely for fun. For similar reasons, I'd encourage every PPL to pursue an aerobatic rating, or at the very least some aerobatic lessons, but the reality is for some people it's hard to justify spending potentially thousands of dollars on something you really don't enjoy, even with the potential of an increase in proficiency it brings.