Because 95% efficacy is twice as good as 90%. And some people are willing to pay the $30 it costs to be 5% likelihood to have a non-functioning vaccine vs. a 10% likelihood the vaccine you just got won't work on you.
It isn't a new thing for what it's worth. Going back to earlier this year it has been looked at, but we don't really have a clear picture on what will shake out.
We need the vaccine to remove the virus from circulation, not just keep us safe from it as individuals.