Due to steric hindrance and +l effect caused by two alkyl groups in propanone.
Detailed Answer:
As alkyl group is an electron releasing group (+ I inductive effect), electron releasing power of two alkyl groups in propanone is higher than that of one alkyl group in ethanal. The reduced positive charge on carbon in propanone discourages the attack of nucleophiles making propanone less reactive than ethanal. Also, presence of two alkyl groups increases the steric hindrance to the attack of nuclophile reducing the reactivity of carbonyl. Thus, propanone is less reactive than ethanal towards nucleophilic addition reactions.