I think the cross is used as a popular symbol for Christ because it is a defining moment in Christian theology that sets it apart from most popular religions. I say "popular" because there are quite a few dying-and-resurrected savior gods in many other religions, too, albeit these religions do not have the same kind of following as Christianity has.
Religions usually have a symbol that 1) reminds it's followers of it's mission/ethics, and 2) visually represents it's unique identity. Jesus taught the beatitudes and the Sermon on the Mount, and these stories in the Gospels are obviously inspiring to read and as examples to live.............yet, his teachings aren't that much different than many other prophets and enlightened beings that graced this Earth.
An olive branch as a symbol for Christianity would be too vague. I could use the olive branch, myself, yet my beliefs are vastly different from most Christian denominations. We aren't exactly in the same camp, there, after the peace and harmony speeches.
A symbol for Christ's resurrection is also, visually, too vague. The cross, despite it's painful resemblence, is specific and represents more than the physical death of the Christian savior.
I may not share the same beliefs theologically as our fellow Christians, and even though I left the religion more than a decade ago, I have much respect for the people who follow Christ. I learned much about life, the universe, and everything when I was Christian.
Peace,
Mystic