Might have to do with modern electrical codes - as electricity in homes has become a standard, and a standard practice for having electricity wired in your home is to have the entire system well grounded. This would negate strikes that come through the electrical grid potentialy miles away or on the power pole right outside your house. But this would do nothing unless your electrical utility connection is conveniently made ontop a conduit that is also at highest point on the structure (most just run a line to the side of your house above your meter).
I know some municipalities have codes mandating lighting rods on structures above a certain height (I think the way it is in NY), but a single-story residential building probabley falls well under it.
Another issue could simpley be enforcement - this would fall under fire and safety or building fire and safety regulations. IE: many places may have lighting rod mandates, but have all-volunteer fire departments (and lack the personel or funding to make sure every house in the county has their lightning rod and that it's properly installed) or lack building departments to aproove plans or the personel in those departments to review them all thuroughly (in the last few years, at least in my industry, plan reviewers are getting younger and less trained and everoyne is becoming more reliant on the superiors - they throw the book at you on some things and overlook everything else).
