Why?

Maybe it's because most other countries use first names, so it's something Asami's accustomed to, but Akihito still follows Japanese customs?!

I have a feeling we'll hear Akihito say Asami's name eventually. So... do you think it will happen while they're doing xxx or for another reason? >:)

This is just my take on it:
Using someones first name or last name for that matter, (without honourifics, san, kun, chan, sama) either implies a sense of closeness to the person(last name would be friends, school mates, while first name would be best friends, family, other halves or husband and wife) OR it can be used as a form of mockery/disrespect, for example when Asami is calling him Akihito at the very beginning of the series it would have been goading, but towards the end where we are now it would be the former that I mentioned.
It's very insulting in Japanese culture to address someone by their first name without a suffix unless given permission, and it's perceived as arrogant and rude. (Though I think our pair are beyond that point.
So while Asami is more forward with this aspect of their relationship, once again Akihito is still a little bit lagging, because calling him Ryuichi instead means he's acknowledged the relation and is addressing Asami as an equal.
some of the charts in here are interesting http://akbok.hubpages.com/hub/JapaneseHonorifics

I agree with the assessment that Akihito is lagging here. He's only just decided to assert himself in this relationship in what? Chapter 50? Before that, he's been so uncertain of where he stands with Asami, he had no confidence in using Asami's given name because he does not understand his place in their relationship.

Remember too how shocked he was at Kuroda calling Asami "Ryuichi," knowing it indicated a closer relationship than was immediately apparent.

That said, I find it so funny that in Haru wo Daieteita that after seventeen(?) years together, Iwaki and Katou are still on a last name basis--in fact, Katou still calls Iwaki "Iwaki-san," even in bed!

I think it may also have something to do with Asami's status. Akihito doesn't think Asami sees him as an equal, so he isn't ready to take that liberty.

i don't think it's about the status. IMO it's more about age difference. Asami is way older than akihito and even when they're already lovers there's still that form of respect needed, unless the older lover himself will say that they don't mind. This is not a separate issue. Actually, if you are a wide BL reader, you'd see that it's actually a common thing. Either they add -san or call the older lover their surname.

In addition to that, I'm pretty sure akihito will be super embarrassed addressing him as ryuuichi-san he'd loathe the very idea since akihito is a tsundere type.

i kinda understand it because we had a similar honorific system in our country. one example: my bro is younger than his wife and sometimes, he calls his wife older sister (not the literal translation in english) in front of other people. <<asian.