Surely the question that should be asked is that why 13 year old Solos were retained in the fleet and the considerably newer ADLs were sold, rather than being cascaded as an operator would normally do? That suggests some dissatisfaction with the product.
As joieman said they are replacing all their single-deckers so there is nowhere to cascade them to (also their E200MMCs are smaller than their Solos so may not have been suitable for such a cascade even temporarilly) plus as 13-year old vehicles the Solos will hold less value and be harder to sell whilst they had a ready buyer for the E200MMCs, they didn't even need to advertise them before they shifted them and given what the new buyer has deployed them on they may well have had a required delivery date that saw them moved out earlier than intended
Also superior is the new Volvo BZL buses that are very comfortable and don't rattle at all. Although somewhere ADL lost the plot was there Coach range being discontinued, instead of modernised and developed. Also the decision to build the ADL E100EV in China can't have helped but presumably it was a competive decision in order to benefit from lower labour costs, and therefore offer either a cheaper product or offer a greater profit margin, or both.
There also seems very few ADL E200's entering service at the moment and this has been another issue, with larger operators turning to Yutong for saloon's and smaller operators turning to Volvo Bus/MCV, that surely indicates issues with that model, and maybe its time its sucessor was brought in sooner rather than later.
Not sure I would describe the BZL, at least in double-deck form, as superior. The lower deck layout is poor due to battery placement so they have a much lower seating capacity than ADL can offer. ADL had said they were considering moving the E100EV & E200EV overseas production back to the UK if demand were high enough but clearly if this closure goes through they won't be able to as they won't have the space.
Diesel sales are a little spotty due to market factors so it is difficult to judge past performance or future potential. Uncertainty due to franchising & funding, big groups sales & a couple of big groups declaring they would only buy zero-emission mean that it is in flux somewhat. E200MMC sales are picking up again and delivery timescales & prices are competitive with Volvo and in the smaller sector where the E200 is strongest it is the best model (at the bigger end the heavyweight Volvo is better for many aspects but it is not as clear to operators as many are making out on here - my employers are very happy with their new E200MMCs and dual-source, they are also very impressed with the Volvos where they are deployed on suitable work).
There is a lot of anti-ADL comments on here, Wrightbus have had many of the same problems or worse and are allowed to have turned themselves around but ADL remain tarred. The classic E200 rattled badly and whilst ADL have never quite got it perfect they have improved with every generation and it needs to be remembered that they have developed what operators ask for in lighter weight & cheap to run models so comparing them to less popular (in terms of sales in the UK) heavyweight models in terms of rattles after years in service is slightly unfair.
ADLs real issues seem to stem from the BYD deal, it was advantageous at the start when they had a fully developed ready to go electric bus into the market early but it tied them to an older design for too long leaving them less competitive and now without a fully developed product ready to deliver when buyers needed them. From how quickly BYD got their range launched vs where ADL were I suspect that it wasn't ADLs choice to end the relationship when they did though you could probably argue they caused it by developing their own range of integral electrics meaning BYD probably felt they were no longer important to them and as they had a developed range that could easily be adapted for the UK that they sold in the rest of the world they could get out with less pain than ADL would feel.