Why, then, was an indult required?
Because it was left up to the individual diocesan bishops to "grant" the annulments. It's the bishops call. Even though Rome never prohibited them, many US bishops refused to grant the indult.
"Even though Rome never prohibited them, many US bishops refused to grant the indult."
Okay, trying to apply logic here.
If a thing cannot be done except by special permission, isn't the default state properly called "prohibited?"