ESTA website has this alert on top in red.
REMINDER: Apply for ESTA no later than 72 hours before departing for the United States. Real-time approvals will no longer be available and arriving at the airport without a previously approved ESTA will likely result in being denied boarding.
Why is that so? ESTA approval previously used to be granted within minutes if not seconds. Is this a generic statement or a policy change?
This press release does confirm the policy change but does not list any reason as to why this was changed.
Due to changes in ESTA application processing, real-time approvals will no longer be available. Citizens of participating Visa Waiver Program (VWP) countries traveling to the United States are strongly encouraged to apply for an ESTA at the time of booking their trip and no later than 72 hours prior to departure. Applicants who apply on the same day of their flight’s departure risk not having an approved ESTA prior to their scheduled departure. International travelers without an approved ESTA will not be authorized to board their flight.
(the emphasis is mine)