The two reasons are delay responsibility and fear of failing.
Delay responsibility is when I feel that my work can be delayed for I am occupied at the moment. This happens a lot, I tend to go online instead of studying. The reason I gave myself is that I was buzy enough for the day, and revision can be done the next day. Unfortunately, the delayed responsibility drag until the last moment and I have to do last minute study. I also spend time on eating, organizing, thinking about unnecessary planning, or random browsing the web.
Fear of failing is inability to start studying due to perfectionism. I am comfortable revising only when I have all the resources I need. This strategy is not good, as I cannot have everything before I start studying. I should realize that there is no appropriate mood and time for studying, you just have to sit down and study. Perfectionism people tend to be procrastinator, they may end up living worse than non-perfectionism people. As they cannot start, they fall behind people that should do worse than them. Perfectionism is not useful when dealing with university style of studying, which is open library learning, where you should expose yourself to different books.