The main why is because of Punk.
Her and Brock had to be last minute panics.
Like I said, Becky was not last minute lol she has been rumoured to be returning to summerslam for a few months now. Her beating Bianca in 20 seconds was just lame ass WWE booking, nothing new to see there.
I am not sure about Brock as he was a surprise return to me, I never read anything about him coming back, the last thing I read was it seemed he wasn’t going to be a part of WWE anymore and was possibly going back to just full MMA. That was in the spring I think I read it, can’t remember exactly, but, it was a while ago, anyway. Never saw another rumour after that about him, all news just seemed to disappear, so, WWE managed to either keep that one a really good secret for a long time, or, they called him Friday night/Saturday morning and did an emergency dump truck of money at his doorstep so he could show up.
I think if they stopped advertising Sasha vs Bianca and put “mystery opponent”, it would have been obvious it was Becky and wanted her return to be a “shock” to people who don’t follow the rumour mill or just didn’t believe the rumours. If they advertised Bianca vs Carmella, no one would give a shit because Carmella sucks ass. People wanted to see Bianca vs Sasha and keeping that match alive kept people watching. I think it’s garbage for people who were excited to see Sasha and Bianca fight again, though.
At the end of the day, we know what we are getting now, just Becky and Charlotte winning for the next 800 years and making the other talent look useless.