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 ????
@manisha I think you've touched on the central issue here. Saturn being a Maraka for ruling the 7th house and a destabilizer for ruling the 8th house can bring these qualities to the 4th house, the 4th house being sensitive for Cancer Lagna could prove troublesome as Moon is Saturn's enemy. In the case of Marilyn, the chart that is being referenced is sidereal and in that chart Moon is malefic and in Capricorn getting a strong aspect from Saturn. In this case Sara Yoga is formed which as part of the description talks about the individual dealing with pain and being a "pained" person. This pain being implicated in perhaps the most vulnerable place for a Cancer Lagna, the 4th house, may cause the person to not be able to ever escape the pain and so becomes quite plagued even if they suffer through it. I think the tropical depiction of her chart shows what was going on better. Saturn is still in the 4th but moves to Scorpio, Leo Lagna, Moon in Aquarius but joins Jupiter and Sun is in the 11th with Mercury and Rahu in the 12th. A tropical search on Kala revels some Cancer Lagna natives with Saturn in Libra; Ava Gardner, Judy Garland, Patrick Swayze, Charles Bronson, none of these have Saturn Retrograde. Charles Bronson was very fortunate to have a full Moon in Taurus; the Moon is afflicted in some way in the other charts and their life stories seem to attest to this much.
Another thing that comes to mind is that 4th house is a moksha house. Saturn rules natural 10th, artha, and 11th, kama and Saturn by nature rules karmic restriction and limitations.
Particular aspect of moksha and the 4th is inner mind of total freedom, inner reservoir (water connotation) where consciousness dwells. Limitation is a direct challenge to freedom and Saturn is drying up what should be a watery place.