"Trust" is the answer.
It's not enough to have all the source code to make a working hard fork good for the community.
You can see this on every new currency born after Bitcoin.
There are some big and trustful names behind Ripple.
Jed McCaleb as example was well known before making the Mtgox exchanger.
He has a very good knowledge about cryptography/math/p2p algorithm, and he had for sure a big address book of other big names that
he trusts.
So Ripple has probably one of best team of cryptographers around the web now. (Bitcoin-qt is another one, Freenet, Tor ...)
As I remember, public speaking, Jed hasn't scammed anyone with his past projects.
So, if there were a 1:1 clone of Ripple, many people will chose the "original".
The ripple-clone need to make/show some very good features to open his own market, good luck