Where I am in the US all our supermarkets suck, and are over priced. Aldi offers a clean environment with inexpensive food at decent non farm stand quality. When your alternative is double the price in a run down store or Walmart. Aldi is a very nice alternative.
I really don’t think it takes much for Aldi to compete in a large part of the US market. Even if they’re not the best because we have so much of the worst in supermarkets.