This question was answered directly by Gordon B. Hinckley, who was the most recent president of the church before the current one. In an article in 2005, he wrote:
Following the renovation of the Mesa Arizona Temple some years ago,
clergy of other religions were invited to tour it on the first day of
the open house period. Hundreds responded. In speaking to them, I said
we would be pleased to answer any queries they might have. Among these
was one from a Protestant minister.
Said he: “I’ve been all through this building, this temple which
carries on its face the name of Jesus Christ, but nowhere have I seen
any representation of the cross, the symbol of Christianity. I have
noted your buildings elsewhere and likewise find an absence of the
cross. Why is this when you say you believe in Jesus Christ?”
I responded: “I do not wish to give offense to any of my Christian
colleagues who use the cross on the steeples of their cathedrals and
at the altars of their chapels, who wear it on their vestments, and
imprint it on their books and other literature. But for us, the cross
is the symbol of the dying Christ, while our message is a declaration
of the Living Christ.”
He then asked: “If you do not use the cross, what is the symbol of
your religion?”
I replied that the lives of our people must become the most meaningful
expression of our faith and, in fact, therefore, the symbol of our
worship.
I know this site prefers explanations over long quotations, but there really isn't much more that can be added to that.