Politicians often have to change course when their campaign promises run up against reality. But when President Trump ran for office, and throughout his presidency, an explicit part of his pitch was that he wasn't like all the other politicians. Trump's "great wall" was a big concrete symbol of that.
Except that now it isn't even concrete. In recent weeks, he has been saying it will be made of "beautiful" steel slats. The way Trump has described the wall has changed a lot over time, to the point of contradiction. That includes how the wall will be paid for.