Short answer: Aside from factors you can control, you may have trouble passing out because your brain is constantly thinking whereas other people may not be. Some people are just born with overactive brains.
Long answer: Lots of factors play a role: consistency of your schedule, diet, how physically active you are, overbearing moods (being really happy or sad), just to name a few. But these are all factors we can control.
Barring the above, some people just have a natural tendency to fall asleep faster than others. Part of this is due to how effectively we can "shut off" our brains. Personally, I had trouble falling asleep because my brain wouldn't stop thinking, and if your brain is busy focusing on various thoughts then it isn't able to turn off and sleep. So the question is, why can some people "shut off" their brain more easily than others? It turns out that some people are just born with overactive brains, thereby making it harder to fall asleep as they drown in all their thoughts.While you may not necessarily be able to permanently reduce your base brain activity level, you can help control it. Focusing on some sort of background noise (white noise, soundscapes, etc.) can help filter out the thoughts. Meditation (especially focus-based meditation) can also teach you to let go of thoughts, allowing you to sort of dissociate yourself from the thoughts that float into your head, which makes it easier to fall asleep.
Additional info: There are many possible explanations of why some people have overactive and underactive brains. A popular explanation relates to the introversion/extroversion personality trait. Briefly, an introvert is portrayed as the quiet and reclusive individual who prefers to spend time doing low-key things, whereas the extrovert is seen as the bubbly and charismatic life of the party. (In truth, introversion/extroversion is a continuum with only few people being at either extreme described above; it is possible to be a "social introvert" for example.) Many people assumed introverts had a slower-than-normal brain activity level (hence why they prefer to avoid social situations) and extroverts had a higher-than-normal brain activity level (hence why they thrive in social situations). But, in the 80's (I think), psychologist Hans Eysenck theorized that the exact opposite is true: introverts actually have a higher than normal brain activity level, and they want to avoid high-stimulus activities (such as parties and crowds) because their brains are already operating at such a high capacity that the extra stimulus is overload for them, and extroverts have a lower base brain activity level so they need to surround themselves with stimulus (such as parties and crowds) to get their brain working at a higher level. (Part of Eysenck's reasoning is a phenomenon where people operate best when they are at some moderate level of arousal, and being over-aroused or under-aroused causes us to perform less optimally. Extroverts are chronically under-aroused so they try to bump their arousal up to their optimal level, and introverts are chronically over-aroused so they want to avoid adding any extra arousal.) According to this theory, then, the reason some people have trouble falling asleep is because they are more introverted: their brains are constantly processing things so it is hard for them to "shut if off" at night. Extroverts have no problem passing out because it is easier for them to clear their heads of thoughts.
But whether or not this particular theory is true, the point remains that some people's brains just don't stop: we say these people have a high need for cognition, which is scientific jargon that translates to "these people think a lot and they often enjoy thinking a lot."
Ritalin , although it acts to reduce the symptoms of ADD/ADHD and causes people to calm down more, Ritalin is actually a stimulant, meaning it increasesyour brain activity levels. Sounds counterintuitive, right? If Eysenck's theory is correct, this works because it brings their arousal up to an optimal level.
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