Because No One Wants to Study.
I get it. We're all busy, and when we're not busy, we don't want to use our downtime to study. Over the years of my own learning, I've found that the hardest step is just getting myself to sit down and work sometimes - I'd rather do a thousand other things.
But studying doesn't have to be time-consuming. In fact, you can do it without taking time away from whatever else you have to do.
Although "real" learning isn't memorization, we do actually have to memorize sometimes: vocab words, formulas, measurement conversions, or special right triangles. But sitting down for a long study session of these sounds about as exciting as cleaning public toilets.
Fear not. I have three solutions that I share with all of my students.
First, find your downtime. When are you doing a menial task that doesn't use any brainpower? Driving, washing dishes, vacuuming, getting ready in the morning - during all of these activities, my hands are busy, but my brain isn't. (Don't worry - I still somehow remember to stop at red lights!) Right before you do this activity, look over a couple of things that you need to memorize and then rehearse them while you do this activity. You can also download podcasts (or easily record your own) and listen to these while you work or commute. For instance, I love the quirky songs produced in the Princeton Review Vocab Minute (free on iTunes).
Second, whiteboard markers and lipstick are your friends! A couple of months ago, I had a student who said that he was awful at memorizing formulas - but he needed to learn the quadratic equation to pass his algebra class. I suggested that he write it on his shower tiles (NOT the grout) with cheap lipstick. Then he could practice it whenever he took a shower (more downtime). His bathroom looked ridiculous for a couple of weeks, but it worked. Women (who tend to spend more time doing hair and makeup) might prefer to write on their bathroom mirror with whiteboard marker. Whiteboard marker easily cleans off of glass, and after a couple of days of looking at that formula or a couple of vocab words, you're likely to remember it.
Third, whenever possible, make it a game! There are several vocab games that you can download to an iPhone or iPod (and I imagine that Droids have them, too, but I don't actually have a Droid to find out for sure). Think about all of the time you've spent playing Angry Birds (or your game of choice while you're waiting in doctors' offices) - you could use that time to master some vocabulary.
The brain can hold seven pieces of new information at once (which is why phone numbers have seven digits!), so don't overdo it. You aren't going to learn fifty vocab words in one sitting. Study seven, and then the next time you sit down, add seven more (being sure to review the last seven). Keep building up the stack, seven words at a time, and you'll find yourself actually *learning* the words, not just cramming them in and forgetting them in a few days.