Because only the US seems to have this guideline. Canada and the EU are not recommending boosters to the general population as pointed out above. It's similar for the flu; afaik, the EU recommends flu shots for ages 60+, not for the general adult population. The US makes a big deal every year on getting flu shots; in corporate media, in shops and outlets, etc.
Edit: I stand corrected on the Canadian front; I had old information.
Edit: I stand corrected on the Canadian front; I had old information.