To me, I thought Wrex would make the conscious effort to try and avoid it. He was battle heavy and not forgiving, but in the end he was tired of his people pointless being brutes and basically going extinct. 

Plus I knew I'd end up having to kill Wrex in order to do it, and I didn't want that. Same with Mordin, though he I didn't know he would die until that event.

Same with the Geth and Quarians. I didn't want to kill off a entire race unless it was avoidable. I did do it with the Destroy ending, because the Reapers had to die. The other options just felt like too tempting lies. But I'd hope the Quarians would be able to rebuild them.

But I'm too much of a forgiving person xD 

I was so sad when I couldn't save people... Like Ashley or Mordin... Or Thane