Several things can limit the maximum potential frame rate. While sensor read on a mirrorless camera can be very fast, theoretically allowing frame rates above 12, the whole read and image processing pipeline has to support the desired frame rate.

In addition to the sensor, you also have the ADC (which may be on or off die, and parallel or not), the image processing unit itself (i.e. its data processing rate), the data rate of the buffer memory, the maximum write speed of your memory card controller, and the write speed of the memory card itself. Any one of these components can limit the maximum frame rate. While image processors used in digital cameras are highly specialized and very fast at what they do, they still have their limitations.

The 1D X, for example, which is capable of 14fps at its absolute maximum, uses a pair of DIGIC 5+ processing units. The DIGIC chips each house eight ADC units and image processing logic, and are capable of processing approximately 250mb/s worth of data. As a pair, the total data throughput rate is 500mb/s. Given that one raw sensor frame from the 1D X sensor is 19.1mp (yes, EVERY pixel in the sensor is read, so while the megapixel count of the final image is 18mp, the sensor has a total of around 19.1mp, including the masked border pixels used for calibration), one single frame at 14bit is 33.425mb in size. Multiply that by 14, and your total raw sensor read data size is 467.950mb. To get 15fps, you would require 501.375mb of data throughput, which is above the capabilities of the dual DIGIC 5+ setup the 1D X has (not to mention that there is going to be at least some overhead above and beyond just the raw pixels, meaning the 1D X is probably pretty close to its 500mb/s limit...a clear sign of fairly careful engineering of the 1D X to keep costs as low as possible while still achieving goals.)

Every camera, including mirrorless cameras, are going to have similar limitations. The 10fps of the NEX-7 is probably limited by data throughput of some component somewhere. Assuming it is 12bit data, then at 24.7mp (the total pixel count) 10fps would require at least 370.5mb/s. The image processor in the NEX-7 is then probably capable of 400mb/s data throughput in order to handle any overhead.

While a mechanical shutter is obviously a key factor that limits frame rate, eliminating it does not necessarily mean you are immediately able to achieve a significantly higher rate...every component in the pipeline must be capable of handling the higher data rate. That means making the whole entire camera more expensive...possibly considerably more expensive. It's all a balancing act, between making a product appealing to customers without breaking their bank in order to elicit the maximum number of sales without losing money. (Which, ironically, is NOT what Sony does...their electronics division has been hemorrhaging money for over a decade, to the tune of over 10 billion dollars...go figure. :P)