Krishna speaks a plain truth here. For those whose minds are set on the formless, the effort is far greater. The path to the unmanifest is attained by embodied beings only with great difficulty.
The reason is straightforward. We live in a world of names and forms. A child recognizes its mother by her face, by her voice, by the feel of her hand. The mind naturally grasps what it can see, hear, and touch. Reaching toward something that has no form, no name, no qualities the senses can hold onto — that demands an extraordinary discipline most people simply cannot sustain while living in a body.
This shloka does not diminish the nirguna path. It simply acknowledges the natural limitation of embodied life. Saguna bhakti — devotion with form, name, and personal love — is the path better suited to human beings as they are.