Why are AI systems almost always feminine?

I don't see any all-over gender bias in AI characters generally.
Boats and ships always get the feminine pronoun, that wouldn't change for spaceships. That would bias your AI selection. Also, various female stereotypes tailored to a primarily male audience come into play, for example, men generally think a female voice is more pleasant. A female AI will not trigger subconscious male-male dominance games. This article from CNN goes into some detail: "Why computer voices are mostly female" by Brandon Griggs, October 21, 2011. This should answer the underlying question.

ImaginaryEvents has some good thoughts. I'd like to add that in Star Trek, the Enterprise computer voice was provided by Majel Barrett-Roddenberry, from TOS until her death. Star Trek has been very defining for the genre, so lots of other fictional computer voices have probably been consciously or unconsciously inspired by her voice acting.

Also, to list a few counter-examples to your premise:

If we look at the names on Wikipedia's list of fictional computers we see that the majority seem to be genderless, with some male names and some female names. That's only looking at the names though.

Some examples of computers I'm familliar with that seem to be definently "male":

• Eddie the ship computer from the Hitchhiker's Guide to the Galaxy
• Marvin the paranoid android, also from the Hitchhiker's Guide to the Galaxy
• Jarvis from the Iron Man movies.
• I.N.T.E.L.L.I.G.E.N.C.E from Team America
• KITT from Knight Rider
• As mentioned already, HAL 9000
• Auto the autopilot in Wall-E, but he's sort of an homage to HAL

I want to say that I read a study done by the US Navy that men react better to a feminine voice during stressful situations, and our auditory senses are more likely to interact with a slightly higher-pitched tone as more suggestive for acknowledgement than that of a more bass-like tone. We see it in movies all the time; the "authorative" figure is some old crusty with a brass voice and an attitude. This is also the same figure that the rebellious type would flaunt against. Now put Angelina's voice in there, and everything's gravy. It's like listening to your mother.

Plus, sex sells.

I find female voices more irritating (as a woman), so I would only have the male version of Siri or some equivalent if I downloaded an AI. David 8 in Prometheus had an incredibly calming voice and I would certainly be happier living with a male AI in the future if I had a choice. But most of the women I know aren't really 'into' technology and wouldn't have much of an opinion either way.

If men are more involved in technological trends then I suppose it would make sense to see more female AI.

Most likely because writers and protagonists are male. This is the case for the AI in Enders Game. However in The Postman, the AIs are male. In the Culture series we don't find out whtether the AIs piloting the giant spaceships are gendered.