Why didn't God the Father answer Joseph Smith's questions directly? In Joseph's version of the first vision given in the Pearl of Great Price, God the Father directs Joseph Smith to listen to his Son Jesus Christ:

I saw two Personages, whose brightness and glory defy all description,
standing above me in the air. One of them spake unto me, calling me by
name and said, pointing to the other—This is My Beloved Son. Hear
Him! (JS-History 1:17)

Why did the Father defer to Christ instead of answering Joseph's questions directly?
Because the the Father gave the world His Son (Isa. 9:6, John 3:16).


It is the doctrine of Christ (3 Nephi 11):


  35 Verily, verily, I say unto you, that this is my doctrine, and I
  bear record of it from the Father; and whoso believeth in me believeth
  in the Father also; and unto him will the Father bear record of me,
  for he will visit him with fire and with the Holy Ghost.
  
  36 And thus will the Father bear record of me, and the Holy Ghost will
  bear record unto him of the Father and me; for the Father, and I, and
  the Holy Ghost are one.

The Father is glorified in the Son (John 14:13, 3 Nephi 9:15, D&amp;C 76:43). Selecting 3 Nephi 9:15 here:


  15 Behold, I am Jesus Christ the Son of God. I created the heavens and the earth, and all things that in them are. I was with the Father from the beginning. I am in the Father, and the Father in me; and in me hath the Father glorified his name.

The Father is the second witness to that of the Holy Ghost of the divine sonship of Christ, and by two or three witnesses shall every word be established (Matthew 18:16).
The principle in the Priesthood of delegating responsibility may be part of the reason as well. The Father does this several times, including committing judgment to His Son (John 5:22):


  22 For the Father judgeth no man, but hath committed all judgment unto the Son



In almost every case in LDS theology, God the Father introduces His Son, who is our Mediator and Advocate to the Father.