analog video vs digital video

Why analog video signal could not get the same high resolution like digital video signal?
In general, a "direct" analog signal will always beat a "direct" digital signal because of fewer conversions in the signal path. I don't think that's what you're asking, however. Both can be terrific if done correctly, but generally, the fewer conversions the signal goes through, the better the picture quality. Take DVD, for example. It's already gone from analog(usually, unless it was originally digitally stored) and converted to digital(for storage on the DVD), so some resolution will be lost. Next, it may be converted back to analog in your DVD player to feed your television, but then your TV might even convert it back to digital(for 2:3 upsampling, resizing of the picture, etc) and then back to analog for the final display. Each conversion process will always result in at least a slight loss of picture information. Even if it's not noticible, errors compound themselves as they travel down the chain. This results in loss of real resolution, however slight.
Now, if you kept the signal digital throughout the whole chain, there would only be one coversion-back to analog so you could see it at the display. Likewise with an analog source, therefore fewer chances to loose resolution. Remember, we see analog, so even filming in digital means resolution is immediately limited to your equipment. The key to good digital video is to get the pixel count so high(detail) that you can't tell the differences apart, much like higher resolution audio.
Now, to end my rambling and sum it all up-the fewer conversions the better the picture. Hope this helps...