Why can't I fall asleep? Could it be depression?

Many reasons. There are a whole host of reasons why you can't fall asleep: too much caffeine, sleep apnea, not sleeping at the right time, a disturbed internal clock, and - yes - depression. This should be evaluated by a sleep expert, and I would urge you to do so. Lack of sleep can be quite harmful.
Yes, but. . . There are so many other possible factors making sleep difficult for you. The fact that you suspect depression is relevant, so be sure to sort it out with a good mental health physician. But depression is not just a bedtime condition, so look more broadly. (and, don't forget that sleep deprivation may be producing both anxiety and depressive symptoms, so the sleep problems could well be primary).