I think this is a problem that will continue to grow. I think that people are taking longer to decide because there are more uncertainties in the decision making process.
In addition, some schools were really slow at providing further information about the offer and I got the sense that students are supposed to find out more at the open house. But this could be several weeks away and for some places, I could have made a decision earlier if my questions were answered earlier. And for some places, it turns out that some answers change by the time the open house happened.
The offer letters I received basically said that they would appreciate hearing from me ASAP but then cited the council of graduate studies (or something) agreement and said there is no obligation to respond before April 15. Based on this, I probably would have waited until the last week to make a decision, because what if I decide too soon and situations change?? But since I did know that waiting could cause others to miss out on a spot (from past experience and this forum), it didn't make sense to hold out on every offer (e.g. safeties) in the negligible chance something could change my mind. But not every student is on here, and the admissions process might not be clear to some students.
So I think it would encourage more people to reply sooner if the letters were more strongly worded so that students understand why an earlier response is preferable. Otherwise, it is like a homework deadline -- why hand it in earlier when you still have a few weeks to go?
I also think it would greatly help if there was a standardized (range of) date(s) for all applications to be due, for decisions to be made, and for students to respond. Also, if graduate programs were as open as possible about the status of applications, then students may feel like they have enough information to make "early" decisions. For example, when an application is submitted, or even on the application information page, the department can give stats such as: average # of applicants, average # of offers made, average # of people who accept the offers, percentage of international students admitted, and the stipend amounts for the last few years. Also, after the first wave of acceptances go out, it would be nice to know sooner rather than later if you're on a waitlist or not, and this email (no snail mail) should contain a line that says something like we received X applications, made Y1 offers while expecting Y2 acceptances, and you are one of Z students on a waitlist. If possible, the student's position on the waitlist should be indicated, but I understand that position depends on who ends up accepting the offer!
Basically, what I'm saying is that students are playing it safe by waiting longer to make decisions. But (some, perhaps not yours) graduate programs are doing a very similar thing by carefully guarding their admissions information as well*. This leads to us students having no idea where we stand so the only thing we can do is wait for more information, which might never come. Ideally, an increase in information from graduate admissions is desired, but at the very least, explaining to students why responding sooner rather than later could be helpful.
*Note: I don't think this information is necessarily secret, because most of my rejections usually give X and Y1 at least. During Open Houses, profs have told me their school's Y2 fractions, usually around 30%, which matches what is discussed here. So it is not really a matter of secrets, but a lack of communication.