Because CMOS sensors can't reset quickly. Why do you think any DSLR has a shutter? If they could do without they would have removed them. Although the name suggests the technology is digital, in actual fact digital sensors use good old analog techniques to capture charge based on light falling on the sensor. This is the bit which can't be reset (drained away) quickly enough when light is still falling on the sensor. The shutter really gives a black time when you can dump charge and reset things before allowing light to build up further charge.

I believe CCD sensors can be more quickly flushed of charge, but it's still not quick enough to do without a shutter. The benefits to a DSLR of not having a shutter would be to be able to sync flash at any shutter speed. This is one area where leaf shutters are good compared to focal plane shutters (as appear on DSLRs). Hasselblad made leaf and focal plane shutter cameras, and the abilty to use the leaf lenses on the focal plane shutter cameras to give more flexibilty on flash sync.

Lenses which cover 8*8 are basically large format lenses which include leaf shutters. Leaf shutters have a couple of problems - limited size, and a limited upper speed. Typically 1/500th is the fastest a leaf shutter will operate, and the limited diameter means you typically are down to f5.6 or f8 as a maximum aperture.

The maximum aperture will limit the speed advantage against a 35mm DSLR or medium format where f/2 and faster is common (f/1 can be achived at standard lengths if you compromise on image quality, say a noctilux). f/1 vs f/5.6 is 5 stops, or 32 times the amount of light. So an f/1 lens vs an f/5.6 will accept 32 times as much light to start with

The usual complaint about fast lenses is the limited depth of field. However, large format at f/5.6 will also suffer this problem as the larger image format will also offer a limited DOF, but in addition, a slow lens. I guess the answer will be to run the sensor at a higher ISO equivalent, make it more sensitive, and hence allow a smaller aperture to be used, but the tradeoff isn't obvious from the specifications

My guess is that this is a technology demonstrator, and will not be available to average punters on real cameras. Saying that an old large format camera with a 8*8 back would be very cool

I use a canon DSLRs, and 1/8000th is the limit. In strong light it can be a limit with a fast prime (a 1.2 or 1.4 prime for example). The best solution is to then use an ND filter.

If you want faster than that, you are best off using artificial light. Flash will stop just about any action, and it is easy enough to setup triggers for the flash to capture events from motion, sound, or electronically delayed triggering from an event. Of course the joy of digital with shots like this is the ability to quickly tell whether you have got or missed the shot, and due to the random nature of say, a drop of water hitting a bowl of water, you can repeat until you are happy with the result.