Learn Hardcore Japanese: sono korokke ha ninki de, yuugata ni ha yoku urikiremasu. - Those croquettes are popular, so they often sell out in the evening.

Breakdown of sono korokke wa ninki de, yuugata ni wa yoku urikiremasu.

Why is 人気 followed by で here?

Because 人気 behaves like a na-adjective / adjectival noun in this sentence.

The で here is the connective form of だ. It links this part of the sentence to what follows.

So:

そのコロッケは人気です。 = That croquette is popular.
そのコロッケは人気で、夕方にはよく売り切れます。 = That croquette is popular, and it often sells out by evening.

In context, 人気で can also suggest a reason: because it’s popular.

Is 人気 an adjective or a noun?

It is often taught as a noun or na-adjective-like word.

That is why it does not behave like an i-adjective such as 高い or おいしい. Instead, it needs forms like:

So in this sentence, 人気で is not a verb form. It is 人気 plus the connective form of the copula.

Why are there two は particles: コロッケは and 夕方には?