I have a few theories:
#1. I honestly think it's over-diagnosed, mainly in kids. Correct me if I'm wrong here, and I'll admit to not doing much research on this specifically, but as far as I know ADHD is one of the only mental disorders you hear about kids supposedly outgrowing. If they really had an actual disorder, would it be that common to just outgrow it? I think people are rushing to label and medicate kids who are having certain problems, instead of looking into the cause(s) and trying to actually fix them...which is basically my 2nd theory.

#2. Labeling and treating it as ADHD, when its caused by something else. (Along the same lines as what skies_ said, but personally I think they said it better lol). A significantly higher percentage of kids now are living with divorced parents than kids several decades ago. There could still be a lot of stress, fighting, dysfunction, etc going on...stress = lack of focus, among other things. It could be a side effect of abuse, neglect, or virtually any other significant stress going on; or even an effect of past abuse/neglect.

#3. Like winterglen said, more demand on kids now than in the past. But there's also a lot more distractions...computers, cell phones, internet, iPods, social media (depending on the age). Even just wondering about an unread text message or facebook notification could be a distraction.

#4. The above-average kid being bored. I don't know how common this actually is, but its something I've noticed a lot personally. I've known a lot of kids who finish their assignment faster than everyone else or just don't do it at all because they see it as boring, and end up causing trouble simply out of boredom. When these kids were given extra work or more challenging work, their ADHD-like behaviors essentially disappeared.

So yeah...just some ideas I had...