The proliferation of maid shows is a peculiar phenomenon that seems to have sprung up in anime in the past few years. The watershed perhaps being Mahoromatic in 2001 (itself adapted from a 1998 manga) and then picking up speed with He Is My Master in 2005 and exploding recently with a bevy of series such as Hand Maid May, Mai-Otome and Ladies vs. Butlers as well as ancillary shows such as Hayate the Combat Butler and Black Butler. The premise invites branching out into the aristocratic area with series like Princess Lover as well as the host-club dynamic with Ouran Highschool Host Club and this season's Kaichou wa Maid-sama!. The fascination with maids always bugged me, especially the recent maid café situation; coincidentally, reading up on the hikkikomori syndrome lead to a theory.
The most obvious explanation is that of a fetish: French maids and the familiar black and white uniform have been a staple of erotic cinema since it began, the subservient nature of the job fosters this objectification just as plumbers and delivery men are stereotypical staples, they combine accessibility with wish fulfilment. It's the subservience that sticks out though and in many of the anime series the maids are recruited through a perceived financial necessity (He Is My Master, Ouran Highschool Host Club) or simply materialise from nowhere (Mahoromatic, Hand Maid May) which absolves the narrative need to justify their existence as a purview of only the plutocratic or aristocratic (Ladies vs. Butlers, Mai-Otome).
Mostly it was the work of Takeo Doi - a now deceased psychoanalyst who coined the Japanese term "amae", rudimentarily "dependence" - which highlighted an interesting point. If you strip away a lot of the discredited, Japan-centric bias to his work (which is commonly clumped together with other works as "nihonjinron") then the idea behind "amae" is the dependence of one person on another, the most common example being that of a child and the mother. More specifically, Doi infers that the relationship is at its strongest when the desires of one are wordlessly anticipated by the other but that the benevolence of the one being depended upon will always outweigh the demeanour of the other. In short: it doesn't matter how arrogant or snotty the son is, the mother will always love and provide for them.
This is sounding a lot like the role of a maid, however the thrill comes from having someone in the dependence role that has no familial ties which removes a great deal of social and emotional barriers. So the maid fulfils dual roles as a willing servant - the master / subordinate relationship - but fulfilling whatever needs may arise, be they sexual or otherwise. One could even go further than the male master - female maid dynamic to suggest the female master - male butler relationships occupies the same parental surrogacy, filling in the absent father role that is commonly seen in series.
It's no surprise that maids are most bountiful - in all senses of the word - in erotic games and anime, it's also not surprising that most all non-porn instances of maids are replete with some kind of smuttiness. Similarly with the foreign influences usually seen with their introduction, I'm not able to speak with absolute certainty but the maid archetype seems to have been absent from Japan until the encroachment of the West; similar roles existed with nannies and so forth, but not the vast social gap between master and servant. This could of course be over analysing the logical result of an intense focus to legitimise and monetise a well known fetish and the saturation and diversification of traditionally, purely erotic series is to blame for the recent spate. How long until the next fetish reaches such critical mass?
Maybe we can ask somebody.