@Pamela515 As you have found out, there are heavy penalties for cancelling. You need to understand how to vet guests before accepting their requests.
In the first incident, you didn't need to cancel and lose your Superhost status- you could have contacted Airbnb, told them she was planning on bringing 12 people to a 4 person listing, and gotten them to cancel it for you, with no penalties to you.
In the second case, you need to send guests a message when they request to book, making sure they have thoroughly read your listing description, have booked for the exact number or guests that they want to arrive with, make sure they are aware of your house rules, etc. You do that before clicking on accept.
In your current case, yes, it's a drag when guests don't answer in a timely fashion and that the calendar gets held hostage, we're all in the same boat there. I'd send one more message, saying that if you don't get a response by XX (whatever the cut-off time is for clicking on accept or decline) you will be declining the request. Then follow through.
And to answer the question you asked, because they are guest-centric and don't appreciate their hosts.