The reason is that there is no greater and more splendid simchah than to gladden the hearts of the poor and downtrodden. Moreover, one who gladdens the unfortunate is compared to Hashem, who revives the spirit of the lowly and the heart of the depressed.
Physical pleasure is necessarily limited.
The human body can tolerate only a small amount of meat and wine. Therefore, in order to maximize the mitzvah of simchah on Purim, the added dimension of helping others was included. Sharing with peers, mishloach manos, establishes a joyful spirit of camaraderie. Giving to those who are needy and cannot reciprocate, matanos laevyonim, creates an even higher level of simchah. Spiritual pleasure on the other hand knows no limits.