Moyes has just simply never proven that he can win at the top level and who is to say that he ever will be able to?
I would basically say such a complete lack of even looking competitive at the top level, in big games in general or cup runs over such a long space of time suggests he is completely proven... just proven not at all capable to win things.
When Moyes was appointed United boss he certainly wasn't good enough to take the job on. It basically relied on him taking the chance with both hands and stepping up a level to start thinking like a proper world class manager.
He deserved the chance after what he's done at Everton. I'd long admired his work there.
He certainly hasn't done terribly at Everton, but everything he did showed quite obviously where his limits were. You don't have to be at a big club to win things.
There are two types of successful managers who really should be the choices for a big job, a proven successful manager, IE someone who has won things before preferably with a bigger team and more consistently, or someone who has never managed before.
There are pretty much loads of first time managers who have done brilliantly at top clubs, and loads of guys who have had success before(often starting at top clubs but not always) who have gone on to other top jobs and been successful. But guys who have managed for 5-10 years without success rarely go on to be successful if they ever get the top jobs.
Capello's first job was Milan, Mourinho was Porto(not as big but he won and didn't stay there long), Benitez's first job was the Real Madrid B team, Guardiola the Barca B team. Wenger's first job was a ligue 1 team and was only there a couple years before moving to Monaco and started winning straight away there. It's not uncommon to name first time managers to top clubs and the winning mentality often starts straight away, be it a league one team and a great cup run, maybe consistent cup runs and promotions but often straight up top league clubs and doing something significant in the first few years.
Everything about Moyes screams, he's had longer than the vast majority of managers in the prem league yet failed to ever make a team look really good. I would say a first time manager would have had a massively higher chance of being successful at Utd than Moyes did.