I can think of two reasons - as Children of Iluvatar it would be unthinkable for one of the Ainur to knowingly cause them harm. The oath would cause the brothers to resist.
Originally Posted by Isilmacil
Reason two - no more kinslaying, enough was enough.
Another possibility - perhaps Eonwe also suspected that possession of the remaining Silmarils would not bring them peace but it would at least put the Oath to rest.
[FONT=Trebuchet MS]"You can't fight the Enemy with his own Ring without turning into an Enemy" - J.R.R. Tolkien, Letter # 81