I think it was fairly obvious that both Laura Prepon and Jason Biggs characters could be transitory and would go away or have very reduced roles past S01. (Rather than lack of work) Both seem to be choosy about what project they take on, and are likely expensive, so it seemed likely that either the production might want to cut their screen time to save costs, or that they would want to move on after experimenting with these characters. Yet it is sad to see her go. I hope Jason Biggs doesn't follow suit soon (his character could potentially serve as a compare and contrast between life inside and outside, if the shows decides to explore that further, but even that idea might be a very small story line or they could do that with a guard).
It is sad. Maybe it was because because Laura Prepon's character is the closest to a short term "happy ending" (or "happy incarceration") situation that we can foresee. The season finale was very dark and depressing, and left Piper quickly devolving (to at least a longer sentence), her chances for happiness (or even relief) much reduced. They can easily scrounge up another love interest for her inside, but it wouldn't have the same backstory (Alex had a "love of her life" quality that can't be easily replaced).