For the same reason that young Will Turner wasn't cursed after wearing the medallion. After young Will is rescued in the beginning of the first movie, young Elizabeth finds the medallion on young Will, and not wanting him arrested and hung as a pirate, takes it and begins wearing it herself. Neither of them were cursed by that act.
It wasn't the simple act of holding, or being given a piece of eight from the cursed treasure. It was the personal choice of stealing one.
- Bootstrap Bill takes a coin (with greed, as treasure) and is cursed.
- Bootstrap gives the coin to his son (who receives it as a gift without envy or greed), who is not cursed.
- Elizabeth takes the coin with the good intention of saving Will (not with envy or greed), and is not cursed.
- Captain Jack Sparrow later takes a coin off the pile (with the bad intention of killing Barbossa) and is cursed.
- Finally Jack (the monkey) steals a doubloon (with greed, as treasure) and is cursed.
Just for @Logarr:
Would that I could.
He produces the medallion, lets it dangle from his fingers.
Do you not know what this is,
It's a pirate medallion.
It's a piece of the treasure of Isla de
Elizabeth gives an infinitesimal shrug, intrigued despite
BARBOSSA (CONT 'D)
Ah, so you don't know as much as you
pretend. Back when Cortes was cutting a
great bloody swath through the New
World, a high priest gave. him all the
gold they had, with one condition: that
he spare the people's lives. Of course,
Cortes being Cortes, he didn't.
He'd've made a great pirate, that one.
Barbossa stands, moves to a shelf. Puts a key to a medium-
sized polished wooden box -- the Captain's chest. Opens it.
So the priest, with his dying breath,
called on the power of the blood of his
people, and put on the gold a curse. If
anyone took so much as a single piece,
as he was compelled by greed, by greed
he would be consumed.
Inside the chest are charts, some gold, a sextant -- and a
few pages of a Mayan CODEX, pieces of tree bark inscribed
with Mayan glyphs. Barbossa removes them carefully, sets
them on the table. Pours over them.
Within a day of leaving port for Spain,
the treasure ship carrying the gold ...
something went wrong. The ship run
aground, every man aboard dead, save
one. He survived long enough to hide
the gold ashore.
Over time, the dark magic of the curse
seeped into the place, making it a
cursed island. An island of death. Isla
As you can read: ...put on the gold a curse. If anyone took so much as a single piece, as he was compelled by greed, by greed he would be consumed.
The curse was not on the chest, but the gold. And for the curse to be "activated" the gold doubloon needed to be taken, with greed as the motive.