To add my opinion to the others expressed here, I suspect the regular Army SA40 divisions had no desire to invade Egypt whereas the B. S. divisions had arrived and the Libyan divisions were raised specifically for that purpose. I base my comment on three things. First, the December 1940 Italian defense of Sidi Barrani includes only the 4th B.S. division at Sidi Barrani to serve as the reserve for the three Libyan forces defending the west and southwest.
There are no regular Army troops anywhere on the line. This, in spite of the fact that two Army divisions were available, one at Buq Buq's water wells (the 64th) and another (the 63rd) south of it and 19 miles SW of the Maletti Group. Neither of these divisions appear to be positioned to defend Sidi Barrani but, rather, Buq Buq. This is shown on the map below:
Second, there are no troops guarding Maletti's SW flank, a flank that includes a road/trail
right across their rear from Bir Enba
. This is the now famous huge gap in the Italian line. Maletti was aware the British had found this road for they had engaged a British patrol on it. This road/trail ran north from Bir Enba in almost a straight line all the way to Sidi Baranni.
The 63rd Division had been been divided into its two brigades, one at Sofafi/Rabia and the other guarding the flank of 62nd division to its east. While defending Sofafi/Rabia was correct in that the brigade covered a road up the escarpment, the brigade to the east should have moved west into this gap and covered the road/trail at Bir Enba. It did not. Instead, the newly arrived 64th Catanzaro was positioned behind the gap where it could defend the water wells at Buq Buq if the British passed through the gap, took the Libyan divisions and 4th Blackshirt from behind, and cut off their water. Indeed! When the British attacked through the undefended gap, they sent an armored column to take Buq Buq.
Which brings me to my third point. Both Graziani and Maletti knew about this southern road to Sidi Barrani. Graziani had ordered Maletti to use it on September 9, 1940 to reach Sidi Baranni and Maletti knew about it again to order it patrolled until his patrol actually met the British patrols on it, losing 100 men and 5 tanks. He then ceased his patrols and had his tanks face west behind his position, the very direction of the British attack. It seems he had a premonition of what was coming.
As a result, not a single regular army division was on the line unless one counts a brigade of the 63rd, which withdrew without firing a shot or even seeing the British. The only regular army division to see combat was the newly arrived (and created) 64th Catanzaro.
In my opinion, the regular army wanted no part of Egypt and, instead, volunteered the "new guys" (the Blackshirts, Libyans and 64th) as the sacrificial lambs, even going so far as to volunteer them their own divisional trucks to get the sacrificial lambs there in time for the sacrifice. It's why the regular army failed to move the 63rd's second brigade to Bir Enba to cover the gap they knew to be there.
Whoever did cover Bir Enba was going to be in a very unpleasant position. The 63rd did volunteer do so and was the first to evacuate the area.
So my argument is that the regular army voted to send the Libyans, the Blackshirts, and 64th to the front as expendable troops using the excuse that it was easier to motorize them as they were smaller units. Yet Libyan and Blackshirt troops were inferior and were never raised as divisions again. All the Italian moves support my position. I await correction.