When I was researching one of the issues I was coming up with getting adequate doses to most populations was getting 100% compliance everyday.
Since it's a fat soluble vitamin the levels stay elevated in the bloodstream about 2-3 weeks after you would even stop taking it.
Thus you can take doses every week or two if you wanted... or every other day etc. as long as they average out to around up to or about 10k IU (or lower if you're concerned about toxicity). So taking 70k once a week is the same as taking 10k once a day.
Basically, it's easier for the laypopulation to take something once every week than to take something everyday. We don't have compliance problems so once everyday is fine for us.