Saturn in the 4th
Not sure if you found the answer you were looking for. But the quickest one I could think of is that in the natural zodiac, the Moon is debilitated in the 8th house of Scorpio. In the case of the Cancer lagna person, the 8th house falls in the sign of Saturn.
The umbrella that everything happens under is about the natural order of things. So even though the Moon might not be present in Aquarius for this particular person, the natural order of things will be the assumption of a debilitated Moon, with the house lord Saturn being exalted in Libra, which in this case will be the 4th house.
That makes it a lot about the 4th house and the debilitation of the Moon, and hence the foundation of life itself. So, Cancer lagna (natural 4th house), Libra (4th house in this particular chart), Capricorn (4th from the 4th), Saturn (lord of the 4th and 5th from the 4th).
We can also look at Aquarius as the 4th from Scorpio, which is the 5th house. But I am still pondering on that one.
As Ernst says, the energies of the retrograde planets are felt much more strongly than in direct motion.
There are many ways to understand why exalted retrograde 8th lord Saturn is worse for a Cancer lagna. I am still working through all the possibilities. This is just one of them.
This might not be the answer you were looking for and mine might not be the right answer, but I am glad you asked the question. I learnt a lot while thinking about it. The answers Angela and Mirela gave were really good ones. I usually feel the need to understand the why of something, so I always go off on tangents 😊