I have asked myself numerous times that. Haven’t you? I mean honestly, mostly it was because I wanted someone else to change, rather than to see my own flaws - but that’s another topic..!
What we all usually feel if someone is not adopting easily is that they are lazy, or not trying hard enough or deliberately trying to piss you off.. In most normal cases it’s not that at all. What happens is that what seems to be laziness is actually exhaustion - and people made tests to prove the point.
In any situation that requires our full focus, or just goes beyond our standard interactions and doings, we get exhausted. The result is then, reacting below ordinary to, what seem to be, normal situations. It’s sort of a balance of inner-strength.
When someone is put into a new environment they are substituting large amount of new, unfamiliar behaviors for old, comfortable ones - and that tends to burn out the self-control.
Example would be, putting down the toilet seat - for the girls it’s normal that it’s down, but for the guys it’s just an extra effort they have to do and they have to focus on doing it repeatedly - what consequently replaces the energy & focus they’d otherwise have for something else. And now imagine, an important life-affecting situation!
Now, in the end it all depends on the individual, the inner strength, the will to change and the amount of focus… but no one can be blamed if they just can’t do it.