Excerpt from an "Unthinkable" review at IGN:
There were several other plot threads that felt a little undercooked this week. Isabel in particular seemed wasted. She was unceremoniously killed by Nyssa early on. After all she’s done to torment the Queen family this season, I would have liked to see a bigger, more fitting exit for Isabel.
I couldn't agree more.
EDIT: allow me to reproduce the answer to a comment I wrote under the Caps & Clips News.
They had so much story arcs going on this season and they introduced so many characters that they had to sacrifice some, and Summer's character was one of them. Of course it paid off in the season finale as everyone was there - Suicide Squad, League of Assassins, Merlyn (also one of the forgotten in the season finale but at least we know is coming back) - to fight Slade.
Arrow writers had to make choices and decide which non regular character would come back next season; between fan favorites Malcolm Merlyn, Canary/Sara Lance and Isabel Rochev, the choice was easy to make. But it is their fault if the Rochev character didn't click with the audience, as we mentioned before.
At the end of the day, Summer got her paycheck and added a recurring role in a successful tv show in her resume, so there's that.