In my opinion it was a shift in priorities. At the beginning of UE4 whole team was mostly focused only on the engine itself. Their game development efforts were minimal, so they could focus on the tools and think what would be best for (at that time paying) engine users and the engine itself. When the engine was mature enough they started developing their own games and many new engine features (certianly not all of them) are driven by their game dev needs what makes sense, especially when games like Fortnite are so successful. Tools were developed to the minimum to meet the needs for their projects and then sometimes left in that state. With UE5 on the horizon it seems engine developers now have enough time to work again on long term goals and from what I heard they are already revisting some unfinished or outdated systems. This is surely exciting time for Unreal Engine and let’s see what they come up with 
(Disclaimer: this is mostly just speculation).
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