Why is the early morning the best moment to meditate?

The best moment to be meditative is now, the present moment. Normally, the first moment you are consciously aware of in the day is the early morning right after you wake up from sleep. When you become meditative at this earliest time, you start to proactively become conscious as your primary act; your energy as a priority starts to get utilized as consciously as possible for your awakening. Then, the more you can maintain this throughout the day (you will become aware of the importance of this the more you meditate), the more you will awaken to who you truly are, the more you will learn to create your life experience consciously.

Whenever you become meditative, it will be beneficial. When you understand enough the importance of this, you will want to become meditative with your first breath as you wake up. Or you will want to be meditative as your last action when going to sleep, or you will want to be meditative whatever you are doing or when you are not doing much. I liken meditation/meditativeness to consciously handling the steering wheel of a bicycle. When we lose our meditativeness / let go of our staying in awareness, or mindfullness as they call it in some circles, it is as if we let go of the steering wheel of the bicycle. It may seem to go OK for a while, but sooner or later we will start to bump into obstacles. The bicycle here represents our energy system/energy. Sooner or later, when a person lets go of being responsible for/as their energy, the energy will be spent subconsciously, it will start to get scattered as the mind subconsciously tries to find wholeness for itself by repeating patterns from the past it remembers. So, in a way, when we start the day by meditation, we get hold of (consciously take charge of) the steering wheel of the bicycle as the first action in the day. (Even if you are getting to the bathroom as the first action, do this meditatively. From the earliest moment that you can become aware of, become meditative. This (becoming responsible for your entire experience of life the earliest time and maintaining this sense of responsibility (meditativeness) as best as you can) will create the best (most smooth) overall result.

Meditation or meditativeness mentioned in this answer refers to any practice that creates more awareness, more centeredness and gradually directs you to the experience of your Higher Self, to the experience of your highest aspects of Awareness, Love and Intelligence of life. Such a practice can evolve over time, towards the awareness of these deeper aspects, provided that it contains focusing on them to a degree howsoever it is formed. The more refined the practice, the more fully the focus will be on these aspects, without rejecting/seeing as separate from oneself any form of life.

The best moment to be meditative as fully as possible is now, always.