- Why is the new model based on a quite old design like the A330, and not the more modern A350 (which could generally be more efficient and have more modern avionics)?
- In addition, I saw that Boeing built an equivalent aircraft, the Dreamlifter, based upon its biggest model, the 747, so... obvious question: why doesn't Airbus base the Beluga XL off of its giant, the already-tested A380? This could offer a much better payload capacity, and, maybe, even the ability to carry fuselage sections of future A380s (which, currently, must be transported by trucks due to their size).
Considering that (one of) the main reason(s) for introduction of larger Beluga XL is to meet the demands of the A350 XWB production, it doesn't make sense to have a modified A350 for the next transporter: Airbus would prefer to sell A350s to customers than to modify them for internal use.
A380 is not supported by airports near most of the Airbus production centers- defeating the very purpose of having the Beluga XL fleet.
In the end, it boils down to which aircraft can satisfy two requirements- payload and runway capability at Broughton. From caan.asia/en:
Airbus wants an aircraft which not only meets high payload capabilities - including capacity to carry two fully-fitted A350 wings - but can operate within airfield landing limitations at its UK wing facility at Broughton, to which it will deliver A350 wing covers. Broughton's declared available landing distance for the shorter runway 04 approach is 1,663m (5,460ft).
Airbus considers this restriction to be too tight for a modified A330-300 or A340-500, while the payload requirement is too high for an A300-600. It believes an A330-200 variant - tentatively designated the A330-200XL - could potentially cope with the landing criteria at projected weights of around 135t, and is the most promising option.
Boeing Dreamlifter, on the other hand, is purpose built for B787 transport- wings and fuselage, while the Beluga transports only the wings. The distances involved in the B787 production (compared to the only European production centers of A350 XWB) means that Airbus can afford to transport only the wings, rather than fuselages too.
Another important factor is the engineering cost and time for designing ultimately a handful of Beluga XLs.
The existing Beluga is based off the A300-600. The A330/A340 airframe is fundamentally an updated A300 and are highly similar, including its cross-sectional dimensions.
Therefore, Airbus simply carried over many of the existing Beluga designs and made minor modifications to others. You can see how similar the Beluga XL and the original model look. This is not a new idea, an A340-based Beluga was proposed in the early 2000s with similar shared parts.
Boeing did not have an existing aircraft to work with, so they had to design the Dreamlifter from scratch, as Airbus would have to do with a new A350 or A380-based Beluga.