-- How that ship number was offered to the pilots of the flight to take
...and...
-- Why the pilots took it?
My guess:
There's an aircraft management system which picks which aircraft is to fly particular flights on particular routes, based on operational optimizations.
This system was not correctly configured with the list of eligible A321s for Hawaii, so it selected a non-ETOPS A321 by chance.
Holes in processes meant that this mistake was not caught by anyone (including dispatchers and pilots) until the flight was already in the air.
First thing to do is to correctly code which A321s are eligible for the route in the aircraft management system.
Second thing to do is to add procedural cross-checks to catch any mistakes.