My recollection at the time was that the A330s were due to be phased out around then but due to new aircraft delivery delays they were kept on. This was supposed to be a short term thing so perhaps these seats were cheaper?

And with a similar pitch, 5 rows of 4 across is a capacity reduction from 5 rows of 6 across on an A333. Maybe they did the numbers and decided losing 2 or 3 rows of economy didn't stack up to maintain J capacity, given how they planned to utilise these at the time?

That said, and I don't like these seats, they have no doubt been in service with QR for a lot longer than expected (even allowing for the A332s having a holiday with Air Italy). Had they known they would have been in service this long they may have chosen differently at the time, but a lot has happened since then.