1. Kuroko's a hygiene freak. He loves to be clean and can't stand anything dirty. Seriously, he would clean a chair three times before sitting down. Therefore, he don't want his manhood to enter an anus, where it's full of germs. I mean, that's where shiit comes from! Gross! Makes him want to puke just the thought of it.
2. He don't have any experience. Kuroko has never been the top and so he don't want to try something new.
3. He loved to receive it rather than giving it. The pleasure of Akashi entering him was just so good that he always cried out in bliss.
4. His job is the easiest. All Kuroko had to do was to just lay there, spread his legs and let Akashi do the rest. Piece of cake.
5. Akashi's orders are absolute. He's the emperor and should have the superior role. Even if Kuroko does think about being the seme, Akashi wouldn't let him. And that's final. No use in persuading Akashi, his words are absolute.
6. Kuroko admits that he's a bit of a masochist due to Akashi's influence. Akashi's technique are just so awesome, the "toys" he brought are just beyond words.
7. Height. Now, Kuroko knew that he's shorter than Akashi so it's just awkward to have a smaller man top a taller man.
8. Pride. He's been an uke all the time and doesn't like the feeling to give up his position any time soon. Plus, if they have reverse roles, the sex wouldn't be as good as it should be.
9. He looks feminine. Semes generally don't look feminine, rather they are the cool and tough type. Kuroko's nothing like that.
10. All the reasons listed above are valid but most of all, Kuroko's content with being an uke. Who wouldn't if their partner is Akashi?