Maybe that was more me getting old and jaded than the games themselves, .
I believe it's a combination of both factors. The flight sims themselves having less built-in immersion and flight simmers getting older and having the "been there, done that" attitude.
Of course,I think the absolute biggest factor is that the genre simply failed to attract a new generation of flight simmers. Anything that fails to attract a new generation of customers will eventually die out.