To capture the information that an analog signal conveys we need to sample the highest desired frequency at twice its frequency.
Analog video has 6MHz as its highest frequency. Nyquest implies a sample rate somewhere around 12 Million samples/sec.
Early digital television equipment included VTRs. Sony wanted this VTR to easily shift back and forth between handling NTSC analog, used chiefly in North America and Japan, and PAL analog used throughout much of Europe and many other places around the globe. PAL had different horizontal line and vertical field rates. So it had its own set of frequency requirements.
A common harmonic frequency between both standards was 2.25 MHz; multiplying that value by six gave 13.5 MHz. That became the SD sampling frequency.
Since half the samples are for luminance and half are for chrominance so each had 13.5 Mega samples/sec.
Combined, that was 27 Mega samples/sec.
If each sample had a value of 10 bits, that led to the SD serial bit rate of 270 Mbit/sec
