Allow your breasts to return to their steady state
During pregnancy the breast tissues undergo significant change due to hormonal effects. You are correct to wait some time to undergo breast reduction after childbirth.
Another factor which would also contribute is if you are breast feeding. Basically, you want to wait for your breasts to return to their steady state, unaffected by the hormonal influences of the pregnancy.
Another factor is your mobility with your new baby. If you undergo surgery too soon, you may be hindered...