Because it ionizes completely in aqueous solution.
Sodium hydroxide is a strong base because it ionizes completely when dissolved in water, i.e. for every mole of sodium hydroxide that you dissolve you get
#"NaOH"_ ((aq)) -> "Na"_ ((aq))^(+) + "OH"_ ((aq))^(-)#
This means that sodium hydroxide solutions have a very high concentration of hydroxide anions, which, in turn, implies a very low concentration of hydronium cations,
So, we consider sodium hydroxide a strong base because this compound is