BOOM! There it is. Disney doesn't want "almost booked the DP". Many people staying for a week don't (or can't) do the math and overpay for the plan. For a single day, the math is too easy.
Also, while I don't have the real statistics, I'll bet that the percentage of off site visitors who spend a day or two at Universal is way, way higher than the percentage of visitors who stay on site. The lure of sticking with WDW is just too strong. So while you eat 99% of your meals on site, off-site visitors who spend a couple of days at US drive their percentage down to 75% at best. There is no debating the point that once Disney has you on property, it has more of your money.