Simple: it's the "bathtub curve" effect. That is, the majority of the interest is going to be right at the beginning and right at the end.

The way these fund raisers are structured, there's a significant bonus for being in the first wave of contributors (in this case, the "Early Sailors"). Once those first slots are sold out, there's really no reason to jump in; you might as well wait and see if some new interesting perk shows up. But of course, if you do really want to contribute, you have to do so before the clock runs out, so everyone who was waiting tends to pile in at the end.

I guess its a game theory sort of thing...