I think this is for a couple of reasons:
1) He wants to feel validated by his base because this is issue is for some bizarre reason their top concern. He wants to feel liked and worshiped because he’s a whiny little *****.
2) He knows he is too stupid to talk about anything else. It’s easy to fall back on a the wall. It’s a simple concept. Immigration is a complex subject, but a wall just sounds good as the only solution.
Because it's the most important issue.
And because he was elected on the basis of it.
In short, because he is the God Emperor who relentlessly puts the interest of Americans and America first.