When people were devising ways to transmit television signals they had only limited available bandwidth, and since everything was analog compression wasn't feasible. So they did everything they could to minimize the information that needed to be transmitted.
They interlaced the signal, so every frame only had half as much pixels. They also split the signal in chroma and luma (basically colour and brightness, respectively) and then lowered the resolution of the chroma with the justification that the human eye is far more sensitive to differences in brightness than differences in colour. This 'subsampled' chroma should then be interpolated at the receiving end (something which many video players don't do or do badly).
Now at the time this made sense, since it was really the best they could do with no sophisticated ways of compressing images. However as this image demonstrates the human eye can definitely notice differences in colour in some cases. And with digital compression it's no longer true that 4x fewer pixels also means 4x fewer data, so while the decision to compress chroma more heavily might make sense, doing this by artificially lowering the resolution does not.
However for some reason chroma subsampling is still used everywhere; it's one of the many leftovers from the days that video signals were completely analog and only displayed on CRTs.
