Main and obvious reason is: manufacturers needs to protect the pro cameras sales.
Everybody knows Canon are full of moire and aliasing and they do not improve the sensor in cheap DLSR for many years. Also there is not earphone output. Do you want a good sensor ? It exists, you just need more money to get it: 5D Mark III. A little more dynamic range and resolution? OK, C100, C300, C500. The price range of these cameras just says the obvious: do you want pro quality? So pay more, much more!
Sony do not give us mic input in cheap NEX cameras, also they are full of aliasing and low light is noisy with a maximum iso 3200. Do you want pro quality? OK, it exists: FS100, just pay more, much more and get the great low light performance and aliasing free.
Panasonic is giving us more, but dynamic range is poor even in GH3, sensor decreased in size, full of noise in low light. Why don't they just decrease the pixel count in the sensor to reduce noise? Because it is a photo camera which needs more megapixels and to keep a division in the market between consumer, prosumer and pro.
Nikon is getting into the game with the D5200, but there is the horizontal noise pattern...
What about the BMCC? Well, the camera itself is not cheap. SSD storage to shoot raw video is expensive. ProRes does not have the same DR. The camera needs rig to handle. Low light performance is poor. Not so portable solution.
So it is smart idea to stop dreaming about perfect low budget cameras. Want or need more? Simple: pay more!
Or use the solutions to improve the low budget cameras: VK hack, magic lantern firmware, neatvideo noise reduction, vaf antialiasing/moire filter, external sound recorder or juicelink preamp in camera, custom picture profiles/styles to increase DR, better lighting, careful grading in post, film grain simulation, sharpening, stabilizing, and of course good script and direction together with tripods, sliders, stabilizers, steadicams, and so on...
Using low budget cameras is "extract milk from the rocks". But remember it was worse in the era of VHS, SVHS, Hi8, miniDV and also HDV with 35mm adapter.
There is a new feature to come, the HDR/WDR/EDR video, already available in Red cameras. But will it be implemented in low budget cameras? And if it get implemented, will it be completely useful or with limitations like the magic lantern firmware approach? So keep dreaming about perfect low budget cameras is not good idea. Deal with the limitations or pay more.
We'll get it, just not from the big boys that have a pro-video department to protect.
Smaller independent manufacturers are really pushing the limits. Only problem is, is that they're not backed by the might of the large corporations, so it's a roller coaster for customers, just like BlackMagic and then dreamers like the Digital Bolex.
Nikon could perhaps save the day, they have nothing to protect, which surprises me why they've not pushed the boat out with R&D to gain a significant advantage and eat into the still world. Unless the whole industry is just one big price-fixing scam, then I can't see why not.
Shame the Nikon mount is about as flexible as steel. I'd like to see them develop a better larger sensor mirrorless solution. I won't hold my breath though.
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