sono korokke ha ninki de, yuugata ni ha yoku urikiremasu.

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

• 人気だ = is popular
• 人気で、... = being popular, ... / is popular, and ...

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.

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:

• 人気だ = is popular
• 人気です = is popular (polite)
• 人気で = being popular / and popular
• 人気な店 = a popular shop

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

They do different jobs.

• そのコロッケは sets the main topic: as for that croquette
• 夕方には marks evening as a time frame and adds emphasis/contrast with は

So the sentence structure is roughly:

• As for that croquette,
• by evening / in the evening,
• it often sells out.

The second は often gives a nuance like:

• at least by evening
• when it gets to evening
• as for the evening time

It makes the time expression stand out more than plain 夕方に would.

夕方に is a simple time marker: in the evening.

夕方には adds は, which often gives emphasis, contrast, or a sense of by that time.

Compare:

• 夕方に売り切れます。 = It sells out in the evening.
• 夕方には売り切れます。 = By evening, it’s sold out / It sells out by evening.

In this sentence, 夕方には suggests that evening is the point by which it is often gone.

No. Here よく means often.

よく can mean different things depending on context:

• 日本語をよく話します。 = speaks Japanese well
• よく行きます。 = often go
• よく売り切れます。 = often sells out

In this sentence, because it modifies 売り切れます, the natural meaning is often.

売り切れる means to be sold out or to sell out completely.

So:

• 売る = to sell
• 売り切る = to sell all of something out
• 売り切れる = to be sold out / to sell out

In this sentence, 売り切れます is the polite non-past form of 売り切れる.

So よく売り切れます means:

• often sells out
• is often sold out

Because 売ります just means sell. It does not include the idea of all of it being gone.

Compare:

• 売ります = sell
• 売り切れます = sell out / be sold out

The sentence is not merely saying the croquette gets sold. It is saying it becomes unavailable because all of it is gone.

Because the subject/topic is already understood from そのコロッケは.

Japanese often omits repeated subjects when they are clear from context. So after the topic is introduced, the rest of the sentence naturally continues talking about that same thing.

A more explicit English-style version would be:

• As for that croquette, it is popular, and it often sells out by evening.

Japanese does not need to repeat it.

It can suggest either, depending on context.

In this sentence, the most natural interpretation is:

• That croquette is popular, so it often sells out by evening.

The で form can connect descriptions in a neutral and way, but it also often implies a reason when that makes sense.

So both of these are close in feeling:

• It is popular, and it often sells out by evening.
• Because it is popular, it often sells out by evening.

In Japanese, the non-past form is often used for:

• habitual actions
• general truths
• regular tendencies

So よく売り切れます means something like:

• it often sells out
• it tends to sell out

This is a normal way to describe a repeated pattern.

その usually means that, referring to something close to the listener, previously mentioned, or already known in the conversation.

The basic difference is:

• この = this ... (near the speaker)
• その = that ... (near the listener / already mentioned)
• あの = that ... over there (away from both)

So そのコロッケ likely means that croquette the speakers are already talking about.

The comma helps separate the two linked parts of the sentence:

• そのコロッケは人気で、
• 夕方にはよく売り切れます。

It makes the sentence easier to read, especially since 人気で connects into the next clause.

Japanese commas are often flexible, so the comma is helpful style rather than something absolutely required by English-like rules.

No. Normal Japanese would usually be written without spaces:

そのコロッケは人気で、夕方にはよく売り切れます。

Spaces are often added only in beginner materials to make the sentence easier to read.