It is not "not wanting to develop a new single aisle jet", it is "not being able to even if they tried really hard".
Nobody is say why, and I don't either, but please look at how many companies still exist that make the most reliable aircraft to date. We had several companies in the 70s and 80s that did, but many went under after a downturn, a crash or because their aircraft weren't competitive anymore. This is continuing. Our current restrictions for reliability, regulations, economy, ecology and competency have become so dire that the tiny overlap of all these goals is rapidly shrinking.
People only accept the most reliable aircraft of their times. Of course. So if anyone botches reliability, as Boeing does now, their market share suffers tremendously, and rightly so. Some aircraft models remain highly reliable and that's what Airbus can currently sell.
However, regulations are ever increasing, because an insane number of bureaucrats in the US and EU depend on more and more regulations. They cannot allow that regulations ever stop tightening, so regulations are never good enough. In a time where advancements in many established fields of technology have slowed down, regulations have sped up to be faster and faster.
A car sold new in 1970 would probable be legal to sell new in 1980, and maybe even in 1990 with slight corrections for safety and emissions. A car sold new in 2020 cannot be sold new in 2024, because the regulations are turning faster than any engine ever could now. We will have yearly regulation changes that make a car from let's say 2026 absolutely 1000% HARAM CHOPYOURHEADOFF illegal to sell in 2027. And new cars after 2028 will be illegal at all, ever, for all eternity, or until this system finally collapses on itself.
For aircraft, this is no different, because the public has been convinced that these are huge environmental blunders to exist and operate at all, and since no one has an aircraft in their driveway, there's little backlash in pork barrel and regime politics that "make aircraft safer and more environmentally friendly", which, according to published documents by WEF itself is "zero air travel for Plebs beyond 2030, at all, ever". (and it's still a conspiracy theory to point out to the official documents of this organization that operates as some de-facto global government for at least the Western nations.)
And then there's competency. You remember how Boeing pledged to get their workforce to become highly diverse? https://www.reuters.com/articl...
That was in 2020. It's 2024 now and aircraft younger than 2-3 years have insane quality control problems that have nothing to do with any new technology or development, but simple things as "consistently tightening all bolts on the aircraft to their correct torque and correctly checking it twice and documenting the check somewhere and reviewing the bolt torque setting process itself, if too many quality checks light up"