Because based on their knowledge of their own children's sleeping habits and their belief that PdL was a child friendly, safe environment, and knowing they were only 60 seconds away from then - they trusted that all would be well. Something which IIRC the AG said they were entitled to do.
''Hindsight is a wonderful thing'' and ''It is easy to be wise after the event'' are both common sayings. That's because humans make mistakes all the time on a regular basis. Why the McCanns have been excluded from the rest of the human race by some - when it comes to getting things wrong - is totally inexplicable IMO.
Gerry's knowledge of his daughter's sleeping habits led him to believe that she may have left her bed as early as 9.05 pm, so that's no defence, it's a good reason NOT to leave her home alone.
PdL has never been shown as anything but a child-friendly safe environment. The dangers I'm talking about were linked to kids being home alone; crying, getting out of bed, falling etc. Nothing to do with the location.
We all make mistakes but few parents make such basic mistakes, especially those who are described as careful, loving and protective by family and friends.
None of it has anything to do with hindsight, foresight is what was lacking.