'The Conners' showrunner Bruce Helford: Why we killed off Roseanne like that

And, reading more on this:

that The Conners would kill off Roseanne actually makes sense following the seeds that were planted in the ABC revival's 10th season. [...] The series previously revealed last season that Roseanne had developed an opioid addition as she popped the pills to cope with her pain and discomfort.

So, Roseanne was already shown to be addicted to opioids. It's not a huge leap from that to her death by overdose.

However, as shown in the fan reactions article I linked, the parallels between the fictional Roseanne character being killed by drugs, and the real-life Roseanne's career being killed by tweets that she made while supposedly on drugs, was too much for them to accept it was a coincidence.