There are some remarkably bad takes here. Many implying this was a simple decision and 'obviously' the wrong one was made.
Why is YPLM automatically considered by many as nearest suitable? It's isolated, limited RFF, limited hospital facilities, limited ATC, etc They weren't flying a Cessna. The AirAsia guys are thinking about all that and also considering Customs, hotels, maintenance and on it goes. Not to mention that the aeroplane is perfectly capable of flying on one engine. Many here are comfortable because they understand what it would be like at Learmonth (or somewhere similar in Oz) But it wouldn't have been that simple to these guys.
Take this scenario:
400nm out of Manilla an engine fails. 200nm ahead is an uncontrolled/isolated airport with few facilities. You've never been to this airport but heard about it and it's listed as an emergency in your company's manual. Manilla is about an hour behind you but that's a destination with all the bells and whistles. About half way back to Manilla there's a small airport you COULD use if absolutely necessary just for a bit of concrete if it gets very quiet all of a sudden.
It's a 'no-brainer' that you'd land at that isolated airport 200nm ahead? OK. Unbelievably, you stuff up the single engine landing (undoubtedly your first ever on the A330 outside a sim) into the isolated airport and end up with a collapsed gear off the end of the runway followed by a fire. Now you're evacuating with little support and virtually no medical assistance. Multiple fatalities. I can see the pprune thread now...