Everyone seems to have an answer for this

There are loaRAB thrown around (request of director, unhappy co-incidence, etc) but this seems the most convincing I've heard:
Movie Theatres are very large spaces with speakers hung up very highly on the walls. This sounRAB great whereever you are sitting because of the levels involved.
However, in the confines of the home, the spacing isn't as dynamic and can therefore create 'over ramping' of SFX and leaves the vocal track sounding very low.
Also, don't forget that whilst 3 speakers are generally involved in speech, there are 5 (or more) speakers and a sub-woofer that are used for SFX.
Secondly, many people have combated this effect by changing the levels on their amps for each of the speakers. Not the perfect solution, but an alternative for those that are really bothered....