I think because they had to leave their home and even though they might be somewhere better it just doesn't feel right to them like its just not where belong or should be.
After reading this excerpt, I would say that <em>the speaker is amazed by America</em> and that <em>he and his father don't feel comfortable with that feeling because they know deep down inside that they don't belong there.</em>
Explanation:
I think it is pretty clear how amazed the speaker is by America when he says: "<em>We stood awhile, </em><em>marveling at America</em>", but somthing inside is still making him feel sad inside, and that shows when he says: "<em>both of us trying hard to feel luckier than we felt</em>". This sad feeling the speaker and his father had I think it is because they loved that place, but still felt they were visitors there, they didn't feel America as their home. This is very clear when the speaker says: "<em>when we backed away, we saw our reflections / superimposed, big-eyed, dressed too formally / with all due respect </em><em>as visitors to this country.</em>"
