At least, that's what I remember hearing---can't really vouch for it and I'd be happy to see someone else weigh in.
Also, my understanding was that Ada was basically forced on the defense industry by the Pentagon, and then there was eventually a backlash, and they dropped it. It may have had more success if it had been adopted voluntarily all along. In fact, it may yet have a resurgence. I know there is at least one serious Ada vendor out there still.
So they went to C? There's some irrational component in there somewhere.
What about Swift? Reference counting has good properties for real time applications. The toolchain is excellent for implementing verification tools. It has fairly high performance, combined with a very large and growing developer base. If someone put together a toolchain for Swift that covered the most useful 20% of features provided by Ada in that context, then such a thing could take over avionics programming.
In that case, "group" may refer to the avionics software industry as a whole.
Uh, what would you use? As far as I know, the choices are: Assembler, Ada, and C. We're talking about avionics code, here.
C++ is not going to help you in this case. Java will be too slow/unpredictable. Real-Time Java is kind of sketch in my opinion (I could be wrong there). Most everything else is totally out.
Actually I'd probably choose Ada... but there are reasons the industry switched away from Ada.