fissingu ha huseiakusesu no genin da.

Because フィッシング is a loanword based on English. Katakana is commonly used for foreign words, technical terms, and words that sound modern or specialized.

In this case, フィッシング usually means phishing in the cybersecurity sense, even though it is written the same way Japanese would also write fishing. The intended meaning comes from context.

A natural reading is:

フィッシングは ふせいアクセス の げんいん だ。
Fisshingu wa fusei akusesu no gen'in da.

Word-by-word:

• フィッシング = phishing
• は = topic marker
• 不正アクセス = unauthorized access
• の = linking particle, here meaning something like of
• 原因 = cause
• だ = plain-form is

は marks フィッシング as the topic of the sentence.

So the structure is roughly:

As for phishing, it is the cause of unauthorized access.

This does not mean は directly translates to is. Instead, it tells the listener what the sentence is about.

の connects nouns. Here, it shows that 原因 is related to 不正アクセス:

• 不正アクセス = unauthorized access
• 原因 = cause
• 不正アクセスの原因 = the cause of unauthorized access

So の often works like of in English, though it has many other uses too.

だ is the plain, casual form of the copula, while です is more polite.

So:

• フィッシングは不正アクセスの原因だ。 = plain style
• フィッシングは不正アクセスの原因です。 = polite style

Both mean the same thing. The difference is mainly tone and formality.

It functions as one compound noun.

• 不正 = illegal, improper, unauthorized
• アクセス = access

Together, 不正アクセス means unauthorized access or illegal access.

Japanese often combines nouns this way to make technical terms.

Japanese does not normally use articles like a, an, or the.

So 原因 can mean:

• a cause
• the cause

Which one sounds best depends on context and translation choice.

That is very normal in Japanese. You often have to infer definiteness from the situation.

Yes, you could, but the nuance changes.

• フィッシングは不正アクセスの原因だ。
  As for phishing, it is a/the cause of unauthorized access.
  This presents phishing as the topic.

• フィッシングが不正アクセスの原因だ。
  Phishing is the cause of unauthorized access.
  This sounds more like identifying phishing specifically, possibly in contrast to other possibilities.

So は is more topic-oriented, while が is often more focused or identifying.

The spaces are just for learners. Normal Japanese is usually written without spaces:

フィッシングは不正アクセスの原因だ。

Teaching materials often add spaces to make the grammar easier to see.

Not exactly.

• 原因 = cause, especially an objective cause of an event or problem
• 理由 = reason, often an explanation for an action, decision, or situation

In this sentence, 原因 is the natural choice because it is talking about what causes unauthorized access.

So:

• 不正アクセスの原因 = the cause of unauthorized access

Using 理由 here would sound less natural.

The structure is:

A は B だ

where:

• A = フィッシング
• B = 不正アクセスの原因

So the sentence literally works like:

Phishing + topic marker + cause of unauthorized access + is

This is a very common Japanese pattern for saying A is B.