Why is sodium carbonate less soluble in water than sodium bicarbonate?
Oops! The question contains an incorrect assumption. But chemists are amazing - they can still answer such a question:

Sodium carbonate is way more soluble than sodium bicarbonate at all temperatures above about freezing. $Na2CO3$ has several crystalline hydrates that are temperature sensitive and make the curve less smooth than most other salts. (I suppose the caffeine is in there to wake us up.)
So, I woke up to the fact that when we calculate the weight of &quot;sodium carbonate&quot; which dissolves at 0 C, it's the decahydrate, whereas sodium bicarbonate is anhydrous, so at 0 C, the solubility of sodium bicarbonate is 0.833 moles per liter, while the same weight of sodium carbonate decahydrate dissolved in a liter is only 0.245 moles.
Now I'm more puzzled by why sodium carbonate has such a complex hydrate system, and sodium bicarbonate doesn't.