Yes we don't talk about sunsundegui and their attempts, I agree about MCV, they went from a builder with a not great reputation, rehashing Marshall bus bodies to supplying the likes of Lothian, I feel in the sense of non-UK bus builders, we may see a chinese contingent eventually end up coming in, but then they did that with rolling stock a few years ago and that was not really taken upon.Wright haven't "went" bust though... They're in recovery and doing quite well with the future looking pretty good for them. In terms of deliveries Wright have actually delivered more Streetdecks in the past 6 months than ADL have E400s and they have a pretty busy order book, particularly for alternatively powered buses.
Because none of those manufacturers really offer a bus that's suited to the UK market. Just look at Irizar's attempts with the I3. While it may be a decent product the interior layout is not very practical, the angled floor towards the rear can create quite a slip hazard when wet and the demonstrator only had 12 forward facing step free seats. For comparison you can get double that on a E200MMC or Streetlite.
The other issue comes with reliability and aftercare. Even Volvo have had some criticism in recent years regarding poor vehicle reliability, particularly from independents when dealing with the complex engine diagnostics systems as they often have to call a Volvo technician out to solve an issue meaning the vehicle is out of use for days/weeks. There's been quite a few operators who were once loyal Volvo buyers who are now switching to far less complex and more dependable Cummins based vehicles.
MCV are probably the only Non-UK bus builder who have properly managed to get a place in the market, however it's not as if they're new to the UK, they have been around for 20 or so years and got bigger gradually over time.
You only need to look at cars and the reputation Japanese and Korean brands had 20 odd years ago to now, Hyundai for instance has one of the best tech focused cabins in their new Tuscon and has got rave reviews akin to some of the German manufacturers coupled with a decent warranty, I would assume that Japanese and Korean buses are quite heavyweight though and probably not really suited to the UK market