Simply put: it's still considered "difficult" content, and NOT mandatory for progression. Thus they maintained the Full Party restriction on it as they have with all else considered difficult content.

I don't HAVE to challenge any extreme primals for any other reason than challenge sake. If I were not a player concerned with gearing up to the highest degree possible, Law tome weapons and gear would still be more than enough for anything I want to do in game thus far. But there IS a percentage of the community that wants more, and we are actually the minority. To facilitate our goals, we find each other; we don't rely on the Duty Finder, and likely never will.

It is not something lazy on SE's part, it's simply a means to regulate optional content, and one that is always lifted with time/play data feedback.