Estimation of Sodium Hydroxide
There are a few reasons. Some of the most important are:

-Oxalic acid dihydrate isn't hygroscopic (it doesn't absorb water from the atmosphere, whereas sulfuric acid does). If a substance can absorb water from the atmosphere, then when you weigh it out, you won't be able to accurately calculate the number of moles used because you don't know how much is water and how much is your substance. This won't be the case with oxalic acid dihydrate.


It's chemically stable when stored. 
As Waylander said (in his comment elsewhere), it's a solid so is easily weighed out.
It has a moderately high molecular mass of $\pu{126 g/mol}$ (as the dihydrate) so any error in the mass weighed corresponds to a relatively small error in the calculated amount in $\pu{moles}$.


As a result, oxalic acid has become a primary standard: So you can use it to make a solution of accurately known concentration to determine the concentration of a solution of sodium hydroxide (by titration).