I would someday like to be a writer. In no particular order, the reasons I don't typically write.

• I'm currently a graduate student, and while my research/teaching isn't all-consuming yet, it does take considerable amounts of time.
• When I do sit down and try to write something, it is usually not very original. For example, a couple weeks ago I wrote "A crow shook down on me". I also tend to overwrite things, as evidenced in that writeup.
• I have a history of starting large projects and giving up after a day or two -- for example, I've tried NaNoWriMo twice, with less than 5% success both times.

Disclaimer: Typical anti-akrasia advice, as seen on LW and perhaps other places, has generally been unhelpful. Writing isn't close enough to being one of my primary goals for me to prioritize it the way I prioritize research.