Mental and/or physical and/or physiological and/or genetic and/or spiritual and/or social factors.
I disagree on social and spiritual factors. You can repress the drive, but it still exists. Social conditioning does not overcome genetic programming.
This is why sometimes repressed drives will manifest in twisted ways, ergo, catholic priests taking advantage of young boys.
I agree on the rest though.
Thus, asexuality is a result of one of three things:
Either something went wrong with your genetic programming.
You have a mental/emotional issue that prevents the drive from manifesting.
Or you suffer a physical disability of some sort that impedes the drive from manifesting.
The only other thing you could do to subvert the drive is to channel that sexual drive into something else, some way leave information behind. After all genes are just information.
Creating art, literature, music, any form of legacy is basically this (I was actually waiting for someone to bring these up, but no one ever did). This is why scientists, inventors, writers, poets, etc... many died happy and fullfilled virgins. They never had sex, but they still channeled that sexual drive to achieve something greater than themselves. This is basically what a child is too... something greater than you, the next generation. That sex drive is that basic inpulse that drives us to do ANYTHING. It's the core mechanic in what makes us... well, alive. It's just a matter of how you channel it, some settle for the simply level of popping out a kid and having a family, other strive for a higher purpose. Sometimes leaving memes behind can have a grear impact on society than genes.
That sex drive is the simplest most basic sort of programming in all life. It drives evolution, it drives everything. And no, I hope people realize I'm not talking about "lol sex" but actually something much deeper than that. Everything we do in life, somehow stems from this simple line of generic code that has driven evolution for billions of years.
Really stop and think about that... how crazy is that?