The close reason system is slightly complicated.
The default off-topic close reason ('This question does not appear to be about sustainable living within the scope defined in the help center.') is only displayed when there are no 2 off-topic reasons. It seems that because we added a custom off-topic reason ('Questions on gardening that don't need to be answered with sustainability in mind are off-topic here.') there are always 2 off-topic reasons, so the default option isn't displayed anymore.
I think it's better to have the default close reason instead of the 'gardening' reason, so I've deactivated the 'gardening' reason until we come up with a better solution.
BTW, people with enough reputation (I'm not sure where the limit is) can see a third close option called 'Other', but apparently this reason doesn't count towards to 2 off-topic reasons for displaying the default option.