1. A global price target is fair to developing countries
It's not completely fair, but it avoids treating India more stringently than the US. India can see that it could become equally rich, since both have the same price target. (This holds for all developing countries.)
2. A global price target is less risky politically
If a country grows faster than expected, it will not have to pay other countries for the right to emit more, it will only have to price the new emissions the same as its old emissions and keep the resulting carbon revenues.
3. A global price target is easier to negotiate
4. A global price target is easier to enforce