Is it explained directly in the show? Not to my recollection. But here's what I think is a plausible in-universe explanation.
The simplest definition of envoy is "messenger". And that's what the rebels and their activities are: a message to the Protectorate and the meths that they are not supreme, not untouchable, and not everlasting. Their precision strikes are visible and unmistakable statements. We are told by Kovacs and others that indulgent, all-powerful meths are the very things they fought to eliminate. In Quell's mind, she might also think of them as a message that eternal life, and all it has wrought, is a plague to eliminate, not a gift to revel in. But it is not until near the end that she reveals this to anyone, so Kovacs is likely the only one that may have opted into that perception.
The Envoy training scenes, reinforced by other dialogue from Kovacs, tells us that they are trained to attract and inspire locals to their aid and cause. Ultimately they are schooled to do this in a ruthless, utilitarian way: that these people are tools to an end, to be discarded, allowed to die, and/or abandoned the instant the situation warrants it. But any of those who do survive will have been naturally brought into the orbit of the rebellion, and may continue to spread their ideas and support for them. The Envoys in this way are something of an elite tactical (terrorist?) team who do the flashy heavy lifting, but the people left in their wake become the seeds for building a wide ranging and enduring counter-culture. In this way the message of the rebellion is carried across the settled worlds. Except they end up failing at that.
In the books the Rebellion never really dies, as it was always spread far and wide, both before and after Stronghold. In the adaptation the Rebellion is evidently snuffed out after the fall of Stronghold. So the message in the adaptation was, apparently, not delivered as well or as enduringly. Perhaps they should have paid a little more attention to actually cultivating the sympathetic locals? A habit of ruthless exploitation for the sake of the mission would just make it easier for the Protectorate to demonize them and change the perception of what their goals were, which is exactly what we see happen.
The reason for that difference would seem to be Quellcrist Falconer herself. In the books she is a mythical figure who might not have actually existed even before Kovacs was born. But in the adaptation she is alive and well and focused more on the singular goal of imposing finite lifespans on everyone with a single act that can be carried out by just a few people, rather than on the goal of maintaining and sustaining a growing swell of discontent and rebellion that could eventually topple things. This is at least well-explained within the adaptation, I think.
In the adaptation, Quell is a scientist who regrets what has been made of her work, and has an eternity to ruminate on it. She places all the blame and responsibility for it on herself. She wants to involve as few people as possible as a result, an imperative she imparts on others in the Envoy teachings mentioned above. But she was not a tactician, or a political scientist, or a selfless guru, or any other thing that would put enacting and sustaining a long-term cultural revolution in her wheelhouse. A precision, technological strike on an achilles heel that makes use of her particular skills and only drags in a small number of people to complete, however, is exactly her wheelhouse. The whole rebellion is really just about her in the adaptation. It lived with her and died with her, because it was a cult of personality that she never gave any real life to beyond herself and her personal demons.
