why different TV technolgy were used before 10 years in different countries?

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Out of the various television systems in use in different countries,india adopted the 625-B monochrome (black and white) and the compatible PAL-B colour systemsMOst European and many other countries are also using these standards also using many other countries are also using these standards. However the majority of available books on telvison deal with american 525 line monochrome and NTSC colour systems.Moreover, in the latest edition of the popular books on htis subject empahsis has totally shifted to colour telvison because of its wide acceptance in all the advanced countries.