To scale The Rift would mean Turbine needing to make significant changes to the entire raid. At the time of release, instance scaling was probably not even on the table. This means nothing about the Rift was designed for future scaling. Therefore all the monsters are not designed to scale, so each type of monster will need to be changed; likewise the loot is also not designed to scale, and still uses the old 'barter' style drops, which means all the loot tables need to be changed and updated, and probably new loot designed for each mob and boss. New quests will have to be designed and translated, and deeds updated. The raid mechanics will need to be looked at since every class has come a long way since SoA, which will probably lead to a split-up of the instance in the same way that Helegrod experienced. Then once all that is done, QA will need to spend hours upon hours testing it for balance and to make sure the changes didn't break anything. As nice as it would be to have a scaled version of The Rift, it is a huge undertaking which will probably upset more players than it would satisfy.

OD on the otherhand was introduced alongside the 'In Their Abscence' cluster which, if I'm not mistaken, was the first group of instances to introduce scaling. Therefore it's probable that OD was designed with future scalingalready in mind. So the mobs and bosses were probably designed to scale, loot probably designed to scale (but I'd imagine they looked this over before releasing the scaled version of the raid) and the mechanics designed for players of different levels. When it came to introducing the scaling version of OD, most of the work was probably already done.