Motivation might be part of it, but I think it's more than just that. There are several underlying problems...
#1 Parents don't spend enough time with their kids. This is partly due to almost all homes now featuring both parents working full-time. When parents get home from their long days at work, they are too exhausted to spend time with their kids to make sure they are doing their homework, and helping them learn.
#2 Kids have more distractions nowadays. When they get home from school, I suspect a large portion of them play games on console or PC, or watch TV instead of doing homework. The lack of parental supervision (see problem #1) is a direct influence on this.
#3 Schools have needed to scale back the curriculum. Because the students aren't keeping up with the normal amount of material (due to #1 & #2), the schools end up having to reduce how much they present in the classes. The result is that kids are taught far less, and often it's only the bare minimum.
#4 In order to deal with the wimpier curriculum, as well as the less educated kids, grading methods have eroded to the point where "just trying" will earn a passing grade. Kids are no longer being challenged to think or perform, but rather they are rewarded just for showing up. With enough "just trying" they eventually get passed thru the system out into the job market. Once upon a time, a high school diploma meant something. Nowadays it's worth about as much as a piece of toilet paper.
#5 NCLB (No Child Left Behind) has corrupted the education system. In an attempt to fix the problems in the schools (#3 & #4), it enforces more standardized testing to try to force schools to improve. The problem is that #3 & #4 are actually symptoms resulting from #1 & #2, so this law does nothing to address the root problem. In fact it is backfiring entirely. In response to the NCLB testing, schools have changed their teaching to concentrate on just what the NCLB tests, instead of giving kids a full education. So instead of actually teaching kids to think for themselves, students are now being taught how to pass the NCLB tests, and not much else. Failing to show steady year-to-year progress on NCLB tests can be a death sentance funding-wise for a school, so it isn't surprising that they are devoting their limited funding and resources to do this.
#6 Finally, my own pet peeve - sports. In many schools, the budget and time priority for sports trumps everything else. When it comes down to practice on the court vs. time spent on homework, sports invariably wins. For too many kids, homework ends up a last priority, and it is often skipped entirely. Personally, I think sports are great, but they should not be a part of school. Schools should be purely for academic learning. Phy Ed can still be part of that education, but it would just be a class on equal ground with science, math, etc. Organized sports could instead be handled at a city or community level outside of school either on week nights or weekends. I know that to many having sports in school is a sacred cow, but IMHO, it is a cow that should be slaughtered for the sake of improving our education system.