why is this number 2.0 is pronounced by two point ''O". ??

Because this is the correct and international way to pronounce. British pronounce 'zero' as 'nought'. All ways are correct like Two point zero is also correct. But if not sure what the listener speaks 'zero', we should pronounce 'two point zero' as 'twice point oh'

This is the way of pronouncing numbers with decimal. if you have 25.60, you pronounce it as twenty five point six zero