I think that 6:30 is after the dinner hour. Around here things like kids soccer games start at 6:30 - so for many people with kids this age they are used to having eaten and being somewhere by then. I remember things like soccer, choir, and various other activities all started at around 6:30.
So although many adults aren't set up to be somwhere after having supper by 6:30 - people with kids are. I would guess that the hostess has become so used to her schedual with kids that she has forgotten what it was like before.