Well, as stated in above posts - they simply can't get "decent" sound from the ever smaller spaces available in modern screens. For most the sound is still acceptable for normal viewing though. Many wanting better for movie watching simply buy the matching set of 5.1 speakers from the TV manufacturer (usually horrible, bass heavy and muffled sound!).
I think "Hi-Fi" style 5.1 systems are actually in decline after massive growth from the late 90's onwards - most dealers are reporting more of a resurgence in 2 channel sound lately. I think part of the reason is that despite the slimmer screens offering weaker sound than the old CRT's they also take up relatively little space, making the mess of Hi-Fi boxes and wires even less appealing (including style systems). So while DVD growth helped promote the benefits of surround sound to the masses, the larger flat screens that made the most of the better picture on offer have actually taken a little away from the cinema experience by diminishing the acoustic part of the package.
From what I can gather on the grapevine it seems that soundbars are likely to be the next audio push from the big electronics firms - which actually makes a lot of sense in UK homes where the majority of screens are still placed in corners (the worst layout for traditional 5.1!). Our replacement for the Aego-T 5.1 system is most likely going to be an active soundbar product - we have a base for the system and processing already, when we've refined the details we'll start on the relatively straightforward speaker part! We're not going for the type that bounces delayed signals around the room since those are fundamentally flawed (too much of the effect depends on the environment in which it's used)!