It’s the advertiser’s choice. When setting up ads via AdWords or other networks, an advertiser will select the audience by geographic location, demographics, language, or myriad other targeting options the network might allow. That makes sense because serving ads to an audience that can’t understand the ad or act upon it is a waste of money.
In Google AdSense you can allow or block ads via its Ad review center, which I find useful for eliminating deceptive ads, such as those with a big button that says “Start Download” — especially if they might appear on a document sharing page. A good portion of my users are not computer tech-savvy, so this is a big deal.