According to the LDS, why is there large amounts of new revelation if Jesus is the fulfillment?

The Law of Moses

Latter-day Saints believe that the Law of Moses was fulfilled in Christ. In addition to the passages in Hebrews that teach this, it is also expressly stated in 3 Nephi 9:

17 And as many as have received me, to them have I given to become the sons of God; and even so will I to as many as shall believe on my name, for behold, by me redemption cometh, and in me is the law of Moses fulfilled.

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19 And ye shall offer up unto me no more the shedding of blood; yea, your sacrifices and your burnt offerings shall be done away, for I will accept none of your sacrifices and your burnt offerings.

20 And ye shall offer for a sacrifice unto me a broken heart and a contrite spirit...

(see also 2 Nephi 25:24-25)

This does not mean that all laws were abolished--Jesus indeed gave quite a few commands Himself, as did His apostles--the technical term "the Law" as used in the New Testament is a reference to the Law of Moses.

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Dispensations

Latter-day Saints believe that the scriptures contain many examples of dispensations of the Gospel--God dispenses truth anew through a contemporary, authorized spokesman. This does not invalidate the revelation given in prior dispensations, in fact, each of the major dispensations in the Old Testament (Adam, Enoch, Noah, Abraham, Moses) utilized both what God has said in the past and what God was saying today.

Why didn't God just reveal everything all at once, once and for all? I propose at least 3 reasons why He did not do this:

1. People weren't ready for it (see John 16:12)
2. People had changed the teachings, ordinances, and covenants God previously revealed (e.g. see Isaiah 24:5)
3. God gives the guidance people need for the time. He told Noah how to make Noah's ark; He told Moses how to make Moses' ark. He didn't tell Noah how to make the ark of the covenant. Latter-day Saints very much believe that God speaks authoritatively today to address questions--some of which are unique features of our time--so that His will & expectations are clear. A good example of this would be the Family Proclamation, revealed in 1995.

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Apostasy, and Restoration

Latter-day Saints see in the Bible & the Book of Mormon the regular occurrence of apostasy and restoration (see a more thorough discussion here). Apostasy, or rebellion against God, can happen at an individual or general level. In cases of general apostasy, God has at times withdrawn priesthood keys from a group of people until the time is right to restore them.

That it is merciful for God to not give a group of people more light, knowledge, and truth than they are ready for, is very effectively taught in Luke 12:47-48 and in Doctrine & Covenants 82:3.

As has occurred multiple times in the past, Latter-day Saints believe that God revealed anew the Gospel and the Plan of Salvation through an authorized spokesperson (aka a prophet) in modern times, ending a period of general apostasy and beginning a new dispensation. We believe that the fulness of the Gospel of Jesus Christ was restored through the Prophet Joseph Smith (e.g. see Doctrine & Covenants 1:17-18)

Appendix

What about other Christians?

We believe there are many good, sincere people among other denominations, and we do not suggest they discard the truth they have. Rather, we suggest that the doctrines taught outside of the restored church of Jesus Christ are incomplete, and some of them are incorrect. I do not say this to offend anyone--I have great respect for many people of other faiths--but to acknowledge what we believe.

What about the unreached?

Latter-day Saints believe that those who did not have an opportunity to learn of the Gospel will have that opportunity in the next life (see Doctrine & Covenants 138). I do not believe anyone will be denied the opportunity (though many may reject said opportunity) to receive every blessing of the Gospel simply because of the circumstances of their birth.

New revelation vs. restoration of what was previously known

Much of what Nicene Christianity sees as "new revelation", Latter-day Saints see as a restoration of things that were once known. A very thorough, scholarly treatment showing that Latter-day Saint teachings on the Nature of God, the Premortal Existence, the Fall, Grace vs. Works, Theosis, Ordinances, Preaching the Gospel to the Dead, Multiple degrees of glory, Marriage, and more, were taught anciently and are evident in the writings of the Ante-Nicene Fathers, is found in The Inevitable Apostasy by Tad R. Callister.

Note, the concept of God as a created being is not a particularly effective description of Latter-day Saint beliefs on Theosis. We believe God is an exalted man; we are of the same "species" as God. We also believe that men are eternal beings. None of us had an absolute beginning, so in that sense none of us were created out of nothing, and there was never a point where we did not exist. We have progressed (e.g. before birth we had a spirit but no physical body), but we never did not exist:

Man was also in the beginning with God. Intelligence, or the light of truth, was not created or made, neither indeed can be. (Doctrine & Covenants 93:29)

Disclaimer: these thoughts are the product of my own study and do not constitute official statements by The Church of Jesus Christ of Latter-day Saints.