I'm sure there's much more detailed financial reasons that we don't see, but one reason is probably competition. Enterbrain/Famitsu have been providing weekly numbers of some type since there were ever numbers to provide. If MC wanted to get a foothold in the tracking in Japan, they couldn't start off with charging for every bit of information. Now Enterbrain/Famitsu and MC also charge for more detailed analysis, used game sales, sales past the top 30/20 each week and past top 100 each year (love the Top 1000 charts out of Japan!), so it's not like they give everything for free. What they do give is great for weekly analysis, though.
One of the financial reasons is probably the cost of getting the data from a geographic region the size of Japan vs. one the size of the U.S. It probably costs more for NPD to get the data from store systems of a much wider area than Japan has. In the same vein, getting sales data for Europe as a whole would be more difficult than Japan, too.
In the end, I think competition is probably the biggest single reason, though. There really isn't an alternative to NPD for tracking U.S. sales. As such, they have great leverage over what they choose to provide or not provide.
Stupid inertia!