Salamun Alaikum
According to the Quran, its reading is actually witnessed
at any time of the day when it is actually read. Therefore, the Fajr recitation would arguably carry no extra 'witness' over and beyond any other part of the day. Verse 17:78 merely confirms the witnessing mentioned in verse 10:61.
010:061"And you are not (engaged) in any affair, nor do you recite concerning it any portion of the Quran, nor do you do any work but We are witnesses over you when you enter into it, and there does not lie concealed from your Lord the weight of an atom in the earth or in the heaven, nor any thing less than that nor greater, but it is in a clear book"Therefore, the more pertinent question is why has the Quran at Fajr been singled out for specific mention? Here I'll share just a couple of possibilities.
- That in the darkness of the night leading to the early morn, one's mind is clearer, distractions less and hence, perceptive faculties to absorb the message far more acute than at other parts of the day. There is some support for this in verse 73:6 where such a period of night has been described as a time when impression is most keen and speech is more certain (watan wa-aqwamu qilan). In the context of the surrounding verses, it is clear that the main subject is the Quran.
- Given that the theme of the verse is salat, and all prayer periods have been covered bar the morning prayer, the Quran at Fajr covers both its reading and provides some overlap with Salat-Fajr which is also conducted at the same time. (Implying some synonymy). This is a time of the day which has also been emphasised in other verses as a time for God's glorification and hence also emphasised in this verse.
I also humbly feel that one should not overlook the spiritual significance of the time of Fajr when light begins to illuminate the sky from a period of utter darkness. This resonates well with the reading of the Quran at this time which itself provides by way of it guidance, a path which leads from darkness into light.
Finally, in my humble opinion, there is no mention of specific witnessing of angels; the presence of night / day angels or special rewards to be incurred with such a recitation at Fajr. These interpretations are usually inferred from extraneous sources.
I hope that helps, God willing.
Joseph.