raikansya ni, satuei ha kinsi da to setumeisimasita.

Because 来館者に marks the person who received the explanation.

With 説明しました, the pattern is often:

• Aに Bと説明する = explain B to A

So here:

• 来館者に = to the visitors
• 撮影は禁止だと = that photography is prohibited
• 説明しました = explained

So に is not marking direction in a physical sense here; it marks the target of the explanation.

は sets 撮影 up as the topic: as for photography, it is prohibited.

So 撮影は禁止だ has a natural rule-announcement feeling, like:

• As for photography, it is prohibited
• more naturally in English: Photography is prohibited

If you used が instead, it would sound more like simply identifying the subject of the statement. は is very common in signs, rules, and explanations because it highlights the thing being discussed.

撮影 is a noun meaning photography, photo-taking, or filming/shooting, depending on context.

In a place like a museum or gallery, 撮影禁止 usually means no photography or no taking pictures/video.

So in this sentence, 撮影 is not a verb by itself. It is a noun being talked about: photography.

Because this sentence uses the plain form before と.

When Japanese reports what was said or explained, the quoted content usually appears in plain form:

• 撮影は禁止だと説明しました
• explained that photography is prohibited

This does not necessarily mean the speaker was rude. It is just the normal grammar pattern for reported content.

In formal writing or speech, you may also sometimes see 禁止ですと説明しました, especially if the speaker is preserving the politeness of the original wording, but 禁止だと説明しました is the most standard grammatical form for indirect reporting.

と marks the content of what was explained.

So:

• 撮影は禁止だ = photography is prohibited
• 撮影は禁止だと説明しました = explained that photography is prohibited

Here と works like that in English reported speech.

It connects the statement to the verb 説明しました.

It is best understood as reported content, similar to an indirect quote in English.

• 撮影は禁止だと説明しました = explained that photography is prohibited

It does not strongly sound like the exact words spoken. It sounds more like the content of the explanation.

Japanese と can be used for both direct and indirect quotation, so context matters. In this sentence, it feels like reported information rather than a dramatic word-for-word quotation.

The subject is omitted.

Japanese often leaves out subjects when they are understood from context. So this could mean:

• I explained to the visitors...
• We explained to the visitors...
• The staff explained to the visitors...

The sentence itself does not specify who did the explaining.

Because 説明しました here is the verb, not the noun plus する written separately in a way that needs an object marked by を.

The object-like content of the explanation is expressed by the quoted clause with と:

• 撮影は禁止だと説明しました

And the person receiving the explanation is marked by に:

• 来館者に

So the structure is already complete without を.

You could have sentences where 説明する takes を, but with content clauses, と is very common:

• ルールを説明しました = explained the rules
• 撮影は禁止だと説明しました = explained that photography is prohibited

It can mean either, depending on context.

Japanese nouns usually do not have separate singular and plural forms. So 来館者 can mean:

• a visitor
• visitors

In many real contexts, English would naturally translate it as the visitors or visitors, especially in a sentence about giving an announcement or explanation.

来館者 is a specific and formal word meaning visitor to a facility/building, such as a museum, hall, library, or gallery.

It fits very well with announcements or official explanations.

Compare:

• 来館者 = visitors to the facility; formal and precise
• 客 = customers/guests; depends on context
• 人 = person/people; too general

So 来館者 sounds appropriate for an institutional setting.

説明しました means explained, so it suggests giving information in a clear or informative way.

If you say:

• 言いました = said/told

that is more neutral and simply reports speech.

But:

• 説明しました = explained

suggests the speaker provided clarification, instructions, or reasons. In a museum or public facility, that sounds natural when staff are informing visitors about a rule.

Yes, but the nuance is slightly different.

• 撮影は禁止だ = as for photography, it is prohibited
• 撮影禁止だ = photography prohibition / no photography

撮影禁止 is a fixed expression often seen on signs. So:

• 撮影禁止だと説明しました = explained that it was no-photography / that photography was forbidden

This is possible, but 撮影は禁止だ feels a bit more like a full sentence being reported. It explicitly sets up 撮影 as the topic and then comments on it.

It mainly helps readability.

Japanese commas often show a pause or make the structure easier to read. Here it separates:

• the recipient: 来館者に
• the content: 撮影は禁止だと
• the verb: 説明しました

The sentence would still be grammatical without the comma:

• 来館者に撮影は禁止だと説明しました

So the comma is helpful, but not essential.