why is NaOH a poor primary standard ?

Answer 1

Because it is next to impossible to secure a sample of NaOH free of impurities. The most prominent impurity is that solid NaOH pellets absorb CO2 and H2O from the air almost as fast as you can weigh it. You CAN prepare some CO2 free NaOH in SOLUTION form but that still must be standardized so it isn't a primary standard.