Mainly because of customer concerns.
Airbus, or any other aircraft manufacturer can propose a bunch of variants in its catalog, but it's the airlines that defines what aircraft are to be developed next. (On a timeline of 10 years up to 30 years - I don't recall the link, but that's Lufthansa Group's explanation.)
Since the decline of the A340s and 747s, and the success of A330s and 777s (mainly fuel saving and engine maintenance), airlines were in a rush to go twin engined while avoiding mass qualification trainings for a new type as long as possible (A350 and 787). Replacing type for the latest generation counterpart is relevant when a) investment is less than predicted gain, and b) the new type matches traveling volume.
The deal is to attract as much customers as possible from a manufacturer's point of view. The way it is now, you can expect Airbus beating Boeing at that game of both new types and neo ones as a kind of revenge over the 777 beating the A340. The only market where Boeing is still strong is on aircraft the size of the 767-300ER, and therefore doesn't need a re-engined neo-like, nor a new type, as long as no other manufacturer is capable of building a next gen version on that market. On any other Boeing vs. Airbus market, they are roughly on a draw (737s vs A320s, 777s vs A330s).
People like to say that the 787 is the successor of the 767. That can be true between the 767-300ER/400ER and a lightweight version of the 787-8, but the comparison stops there, and the 787-9 is likely to get more orders than -8 anyway. In terms of size, the 767-300ER/400ER and 787-8 are very similar. But when you have a look over payload, range, cabin width and fuel consumption, there's a huge gap, and that's what matters for airlines, not size or overall shape. Remember, the 777 was also supposed to be the successor of the 767, but ended on a completely different market.
In fact, because Boeing already has the 777-200ER/LR, it doesn't make sense to launch a successor while the 777 is still strong. That's also why the future 777-X are larger, to make room for the 787-9 and -10, and also why the -10 is supposed to be launched as late as possible.
Airbus, on the other hand, had to drop the A340 program earlier than expected, despite the launching of the -500 and -600 versions. A330s cannot be upgraded like the A340 without heavy structural changes, that's where the A350 enters the stage, and why the A330 is still a good candidate at being re-engined and improved in terms of payload and range. We can say that Airbus were always late to take the lead over Boeing, but that's the result of being as diverse as the countries involved, and the absolute need of a success in bringing the first full length double-decker widebody airliner.
Boeing also made some weird decisions, like the 747-8I (not the freighter), the 767-400ER and the 787-3 (now cancelled). The 747-8I were supposed to seduce Chinese market, and airlines that were historical customers of the 747 production line. But by the time the 747-8I entered service, twin engined long range airliners already took the lead, were proven more fuel efficient, cheaper in maintenance costs and much more versatile, which match with Boeing philosophy towards the point-to-point theory. Filling the gap between the 777-300ER and the A380 wasn't that good of a move. The 767-400ER was neither much more profitable than the 767-300ER, nor the 777-200ER on their respective routes. The 787-3 obviously aimed the Japanese market (and Chinese domestic routes) where 737s/A320s and 767s were still going well. Initially postponed after 787-9 development, it was finally cancelled. Actually, the current Airbus position about the A350 -800 is quite the same as Boeing's back then with the 787-3. Difference is, the A350-800 is larger than the 787-3, and is the direct competitor of the 787-8 and -9, and depending on travel trends, probably a strong opponent to the 767-300ER.
Knowing all that, Airbus wouldn't risk shorter or longer versions of the A330, and opted both for one type that can be stretched (A350) to fill the void created by the demise of the A340, and current A330 by increasing the operational service lifetime with the neo version. Next is up to travel trends, which is the most difficult part to estimate...
Having the latest iPlane is seducing, but not everyone can afford it.
