The question "Why is the A330 so unpopular?" is somewhat wrong and misleading.
The A330-300 didn't have the success, Airbus had hoped for. So far out of the 300-350 ordered A330 (don't have the exact numbers right at hand) more or less 50% were for the -200, but in only something around 4 years.
Let's take the year 1999 and 2000. In 1999 there were 19 orders for the A332 and 13 for the A333. This year,
69 for the A332 and 28 for the A333. I think, the reason is the better range of the A332 and apparently its overwhelming operating costs. Both factors allow airlines to use this aircraft in a very flexible way.
Now, what you meant was probably, why the A330-500 is so unpopular. Well, the most important was already said. LH, Hapag Lloyd and SQ want a replacement for their aging A330/A310, but the proposed A335 - which is developped out of the A332 - has a too long range and is somewhat too heavy.
Hope this helps
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