Is there a particular reason for this? Is Lothian avoiding Dennis/Wright intentionally?
They have a long standing, successful relationship with Volvo that started at the Leyland takeover with batches of Volvo Olympians and Volvo became the defacto chassis choice consistently since the 2005 B7TL/Wright order.
The relationship with Wrights soured in approximately 2017/8 and whilst I don't have insider insight the most commonly cited factors are the issues with the Streetair EVs and lack of warranty support. The build quality on the 2017/8 intakes of Wright bodied B5TLs and B8Rs is also referenced too as it was shocking.
Wrights has essentially changed hands since then but the Wright Electroliner demonstrator had a torrid time when it was at Lothian and that would on the face of it appeared to only further fuelled the pessimism within Lothian that Wrights cannot be trusted again.
With the exception of 291-4 which as mentioned before were at least part funded by Scottish Power they haven't bought an ADL integral since the last of the 2004 Trident/Presidents - I don't know why, they just fell out of favour.
They appeared to throw ADL a bone with the B8L/XLB and B5TL/MMC batches but both were one offs. ADL have a growing reputation for not wanting to body other companies chassis' - so it's hard to know if it's Lothian not ordering them or ADL not being prepared to make them?
Volvo has set up the relationship with MCV so that problem has to a degree gone away as Lothian has gone with that for two big orders now (B8Rs and the BZLs)
Lothians preference appears to be "Volvo unless it can't be" and that's presumably based on having a good overall ownership experience with a massive fleet of them.