To me, the answer is very simple but once again, let's go back in history and my beliefs a little.
Baha'is believe in something called "progressive revelation". 'Abdu'l-Bahá said:
Religion is the outer expression of the divine reality. Therefore it must be living, vitalized, moving and progressive. If it be without motion and non-progressive it is without the divine life; it is dead. The divine institutes are continuously active and evolutionary; therefore the revelation of them must be progressive and continuous.
Religion is ONE and progressive. All Messengers of God have come from God and come praising the Last and the Next. They do not come to humanity and dismiss what has already been revealed; they rather come and exalt the position of the previous Messenger and His Teachings and Laws and build upon the spiritual laws (those that don't change like killing is forbidden and God is one) and add more that are appropriate for our time and age as well as bringing to us changes to those social laws we are so in need of. Imagine we still lived by "an eye for an eye"!! There would probably be nobody standing! Therefore, the social laws are required to change according to the needs of the time.
Let's think of this in a simpler way using the analogy of the school and its teachers. Think of the teachers as the Messengers of God and the students, humanity. All teachers have gone to school and university and are all endowed with the knowledge to teach everyone from grade 1 to the end. However, a teacher cannot teach year 8 maths to a 2nd grader! Imagine the confusion! Humanity is ever-advancing and so is our capacity of understanding. To state that one teacher is better than the other or that one teacher knows more or is superior is completely wrong.
"The Bahá’í worships not the human personality of Bahá’u’lláh, but the Glory of God manifest through that personality. He reverences Christ and Muḥammad and all God’s former Messengers to mankind, but he recognizes Bahá’u’lláh as the bearer of God’s Message for the new age in which we live, as the Great World teacher Who has come to carry on and consummate the work of His predecessors." -J.E. Esslemont
And the reason I don't have parties during Christmas is NOT because I don't believe in Jesus Christ (I do) but because if I was to celebrate His birth then I would have to do the same for all the Messengers of God from the past! And that's just too much partying if you ask me...