2) The high speed electronics and extra memory required to support a high frame rate would increase the price, perhaps substantially, making it less competitive.
That is the main reason; for several years now, there has been clear trade-offs between frame rate, pixel count, and cost. It seems that at a given state of technology and a given cost, there is a limit on how many pixels can be read per second, "MP/s".
- The original 1Ds had a rather slow frame rate (for an EOS-1 model) of 3fps, while the cheaper 1D was 8fps; clearly Canon was not choosing to hold back the frame rate of their top priced product, but was limited to reading about 33MP per second in each camera.
- The Mark II versions follow the same pattern, but with new Digic II processors raising the rate to about 66MP/s.
- The Nikon D1h and D1x had the same speed/pixel count trade-offs.
- The Nikon D2X has the same trade-offs in the same camera: 12MP@5fps or 7MP@8fps, both about 60MP/s.
- The 20D and 5D follow the same pattern, at 40MP/s: thus the far more expensive 5D has a lower frame rate. (This makes me suspect marketing BS when Canon says the 5D has the same Digic II processor as the 1D Mark II models; same Digic II design yes, but in a slower, cheaper version, as also used in the 20D.)