Question

why total income is not such an useful measure for comparing countries?

Answer 1

Because it does not gives a correct picture of a  country. With averages we cannot find differences and similarties among the two countries , two persons or two things.It is only used for comparing which is not enough.For example a student differ in health, talents , achievement's, studies. A healthiest student may not be the studious one. 

Answer 2

because all countries don't have same population if a country has more population it will have high total income, but doesn't have good standard of living