The first reason is because checkboxes are not actually binary. Binary means only two states, on or off. Checkboxes default to a "never been checked" state where neither on nor off is selected. It's not possible to have all new contacts automatically have the checkbox property start as "off." Therefore the only people with the check box set to off would be people who at one point had it set to on.
A date picker can be truly binary, as you can test whether the date "is known" or "is unknown." Therefore it's a good way to approximate the binary functionality of infusionsoft tags. As an added bonus, which is probably the most important reason, it' easy to see the date they received the property, as that's what the date stamp will indicate.