Lack of interest is my guess. That plus someone in Apple imagining that it shouldn’t compete with Adobe Lightroom. Aperture, during its peak, was well liked by users. Many even liked it better than Lightroom.

Even the iPhoto of that time (on iOS) had features that Photos doesn’t have till today (I recall buying iPhoto that allowed erasing things from a photo or blurring things from a photo just by using one’s finger).