A lot of Reasons, but mostly due to the extremely skewed concepts of Morality that pervade the world of Altered Carbon as a whole
Double-Sleeving in Altered Carbon is weird. It should, from a surface-viewpoint, be something that is viewed as a good thing, as it would be incredibly beneficial to the status quo (creating a surplus of labor to ensure a massive wage gap between the Meths and the Grounders), improve productivity, et cetera. In fact, in similar and derivative universes like Eclipse Phase, "Forking" (ie: Double-Sleeving) is fundamental to the economy and is incredibly normalized. But the views of Protectorate society on Double-Sleeving in Altered Carbon treat it as a massive ethical sin.
The first and most logical, rational reason is that there is already a deficit of Sleeves (and ability to sustain Sleeves) in Protectorate Society, compared to the number of Stacks. There are far more Stacks than the Earth can support sleeves, and many other Protectorate Planets have similar issues, which may or may not be exacerbated by arbitrary factors like deciding that specific planets are "resort worlds" purely for Meths, which means that these worlds can't be allowed to sustain more people because doing that would mean that Meths wouldn't have pretty planets as their exclusive clubhouses anymore.
A second reason, one which would often be cited in-Universe, would be the Legal difficulties of having multiple sleeves in your clones. It would be difficult to distinguish legal responsibility in such an environment in a way that was, again, easily corruptible by Meths to suit their needs.
But there is another reason, which boils down to the fundamental understandings of Human Life in Altered Carbon, and the incredibly skewed systems of morality that underpin the universe. I'm going to go ahead and make a statement here which seems (based on some opinions I have seen made about the series and its Netflix adaptation) surprisingly controversial+:
Altered Carbon is a Dystopia. It is not supposed to be viewed as an ideal society. This is evident in the constant themes of stagnation, human degeneracy, and unchecked squalor in the series despite the technological advancement of the setting.
And, as a Dystopia, Altered Carbon has very skewed perceptions of human existence. A "human" in Protectorate Society is Digital Human Freight. It is not a body, a body or "sleeve" is merely the property of whomever owns
it. Clones are not considered Humans, because they aren't uploaded to DHF and placed in a stack (that would defeat the purpose of clones as a "free" source of bodies to make sleeves which are compatible with the Egos sleeved into them) Clones are placed in Sensory Deprivation Tanks to ensure that they don't start becoming too sentient, both out of a society that views Clones as less-than human, and because it impairs the ability of DHF to re-sleeve into a clone with minimal Ego deterioration, which is the whole purpose of using Clones to resleeve.
This assumption that a "person" is Digital Human Freight, and nothing else is worthy of Human Rights, inherently clashes with the concept of Double-Sleeving. Double-Sleeving destroys the illusion of the sanctity of DHF as the metric by which a Human Life is judged. If consciousness can reproduce outside of Birth, it destroys the social systems on which the Protectorate is built. It leads to big questions about the very nature of consciousness. It is fundamentally dangerous to the social order of the Protectorate, and society has absorbed this order very well, so much so that they treat people who would go against this as if they were worse than people who destroyed consciousness by intentionally destroying stacks.
+: I mean, from a subjective standpoint, I felt this was made clear from the incredibly grey conflict between the Protectorate, motivated by financial and political reasoning to ensure that Stacks continued to exist and to commercialize human existence, and the Envoys, motivated by fanatic and extremist views regarding the problem of DHF and a fetishism of Death, resulting in both sides attempting to Genocide each other; or from the very term Digital Human FREIGHT denigrating human consciousness to the same level as "baggage".