I think it's a combination of attitudes, culture, language, poor ad targeting, geo targeting, etc.

One of my sites was seeing very heavy use by Romanians, yet their adsense pageviews were quite low and I was getting only a penny or 2 each day from them through CPM ads. It was obvious the majority of them were blocking adsense.

I allowed it to go on like that for a while because, while they earned little for me, they did create backlinks that brought more East Europian visits.
It finally got to the point where they were 90% of traffic but less than 1% of earnings, so it wasn't worth continuing.

I created a top banner in Romanian carefully explaining to them how their continued blocking of all ads would result in closure of the site if it continued. The message included info on whitelisting adsense and quotes from adsense itself about default blocking.

I think the message got through. It wasn't a huge improvement but at least CPM earnings were up enough to cover expenses.

You have to be very careful with the wording if you try this though. There's a fine line between education and soliciting. A poorly constructed sentence or bad translation could be seen as an adsense TOS violation.