In short, the latest batch of Reid’s now-controversial writings espoused positions that were at worst hyperbolic versions of positions with considerable support among the Left at the time, some of which continue to have significant support.
Accordingly, absent the dishonesty, the Reid kerfuffle was preordained to end with forgiveness. The modus operandi of the Left is to continue pushing the Overton Window on its issues as far as it can, as fast as it can. More mainstream Democrats and liberals trying to maintain political viability in a given moment often cannot go as far.
For example, in 2008, the Left had to tolerate Obama and Clinton not supporting same-sex marriage. Both were allowed to “evolve,” just as Reid now claims she has. She lied to avoid excommunication, but the Left must be willing to engage in a constant, vaguely Maoist cycle of political re-education and contrition. Modern progressivism is an ouroboros, but the creature cannot consume too much of its own tail at a time, lest it kill itself.