But...but...but...but.... [excuses]
Aren't conservatives always telling everyone else to stop playing the victim and pull yourself up by the bootstraps and take responsibility for yourself? Funny how they don't practice what they preach.
He had 2 years of a Republican Congress and got nothing done. Yes, the Congress without being Republican-controlled would not give him a blank check to do just anything he wants. He and everyone in the nation knew this beforehand. So why make the promise when you know this could (and likely will) be a problem? Why make a promise you can't keep? You do that when you don't really care about keeping promises and just want to tickle the ears of people to get votes...or appeal to racists.
No, THAT's nonsense. You can definitely break a promise if it is made impossible by outside circumstances. It's still a broken promise. People who are wise don't make promises they can't keep. And I could see a case for extreme external circumstances - someone gets killed, some catastrophe happens, etc. But what you and Trump and the other Trump supporters are blaming is "the Democrats". Well, that's not an external, unusual circumstance. The Democrats were known to be there all along and known not to just go along with whatever Trump or the Republicans want to do.
To use your example, if an athlete promises to win a competition and then fails to do it because the games are canceled then, yes, that is an extenuating circumstance. He still broke his promise but it's understandable that it was some unexpected, unusual circumstance. Most people will accept that excuse. But if he fails to win and then says, "But...but...but...my competitors wouldn't let me win!!!!" then nobody's going to buy that as a valid excuse.
So knowing about the Democrats being there why did Trump make the promise? Because he didn't really care. He knew he'd fail and break his promise. It was just red meat to appeal to his followers to appeal to their xenophobia. And it worked, for that ulterior goal. But he FAILED at building his wall and he broke his promise.
He both failed to build the wall and, in doing so, broke his promise.