To cut a long one short...
The A333 is about 10t lighter than the 772ER/772A and burns less on short haul routes. The 772ER has a longer range and higher max payload so is more profitbale on longer routes and obviously can reach more city pairs.
only have the 772A because they can't sell them on at a reasonable price due to their a small cargo door which restrics the 777's cargo handling vesatility. The only other airline to operate 772A's and A333's is TG
, but they have high density 10 abreast seating so the max pax difference of the two planes is quite different.
bought the 777 because they had a lot of deposits for Boeing aircraft (originally 747's but they were allowed to convert them) which they were set to lose if they didn't buy anything. They already had the A343 in the fleet but the 777 came with a better payload/range so instead of buying more 747's - which they had too many of and were inflexiable - they bought the 777 instead. The two planes now have slightly different roles in the fleet.