Learn Hardcore French: Marie est déçue parce que la réunion a été annulée au dernier moment. - Marie is disappointed because the meeting was cancelled at the last moment.

Déçue is agreeing with Marie, who is feminine.
The past participle déçu (from décevoir) is used here as an adjective, so it must agree in gender and number with the person:

masculine singular: déçu – Paul est déçu.
feminine singular: déçue – Marie est déçue.
masculine plural: déçus – Paul et Marc sont déçus.
feminine plural: déçues – Marie et Sophie sont déçues.

Why is it Marie est déçue and not Marie a été déçue?

Marie est déçue describes her current emotional state: Marie is disappointed (now).
Marie a été déçue (passé composé) focuses on the event of her being disappointed at a specific moment in the past: Marie was disappointed (at that time).
In the original sentence, the speaker wants to say that right now Marie is disappointed because of the cancellation, so the present tense est is more natural.

Why is it parce que and not some other word like car or puisque?

Parce que is the neutral, all‑purpose way to say because, and it works in almost every context.
You could say Marie est déçue, car la réunion a été annulée au dernier moment, but car is a bit more formal and is mostly used in writing.
Puisque means since/as (given that) and is used when the reason is already known or obvious to the listener; here we’re just giving a straightforward cause, so parce que is the most natural choice.

Could I use pour que instead of parce que in this sentence?

No. Pour que means so that / in order that and expresses purpose, not cause.

Parce que = because (gives a reason)
Pour que = so that (gives a goal/purpose and requires the subjunctive)

So Marie est déçue pour que la réunion… is incorrect. You need parce que to express the cause of her disappointment.

Why is it la réunion a été annulée? Could I say on a annulé la réunion instead?

La réunion a été annulée is a passive construction: has been cancelled / was cancelled. It focuses on the meeting itself, not on who cancelled it.
On a annulé la réunion au dernier moment is an active sentence meaning They cancelled the meeting at the last minute.
Both are correct; French often uses on in everyday speech, but the passive form in the example sounds a bit more neutral and formal.

What exactly is a été annulée? Why do we need both a été and annulée?

A été annulée is the passive voice in the passé composé (a past tense):

a été = has been / was (auxiliary avoir
- past participle of être)
annulée = past participle of annuler, agreeing with la réunion

So la réunion a été annulée literally means the meeting has been cancelled.
If you said la réunion est annulée, it usually means the meeting is (now) cancelled as a current state, not focusing on the moment of cancellation.

Why does annulée also have an -e at the end?

In the passive voice (a été annulée), the past participle must agree with the subject of the verb.
The subject is la réunion, which is feminine singular, so you add -e: annulée.

masculine singular: Le cours a été annulé.
feminine singular: La réunion a été annulée.
masculine plural: Les cours ont été annulés.
feminine plural: Les réunions ont été annulées.

Why is it la réunion and not le réunion? How do I know réunion is feminine?

In French, each noun has a grammatical gender that you generally just have to learn with the word. Réunion happens to be feminine, so it takes la.
When you learn vocabulary, it helps to memorize it with its article: une réunion, la réunion.
There’s no fully reliable rule for all words, but many nouns ending in -ion (la nation, la question, la solution, la réunion) are feminine.

Why do we say au dernier moment and not à le dernier moment or à la dernière minute?

Au is the contraction of à + le before a masculine singular noun. Moment is masculine, so à le moment becomes au moment → au dernier moment.
You can also say à la dernière minute, which is a very common synonym meaning almost the same thing: at the last minute.
Both au dernier moment and à la dernière minute are idiomatic; the example just happens to use the first one.

Could I leave out the article and just say réunion a été annulée?

No. In French, a noun almost always needs a determiner (article, possessive, etc.).
So you must say la réunion, une réunion, sa réunion, etc.
Saying réunion a été annulée without any article is ungrammatical in standard French.

Do we need the subjunctive after parce que in this sentence?

No. Parce que is followed by the indicative, not the subjunctive, because it simply introduces a factual reason.
So parce que la réunion a été annulée correctly uses the indicative a été.
Subjunctive appears after certain conjunctions of purpose, doubt, fear, etc. (like pour que, bien que, avant que), not after parce que.

How is this sentence pronounced, especially déçue and réunion a été?

Approximate pronunciation in IPA:

Marie est déçue → [ma.ʁi e de.sy]
parce que la réunion a été annulée au dernier moment → [paʁs kə la ʁe.y.njɔ̃.na.ete any.le o dɛʁ.nje mɔ.mɑ̃]

Two tips:

déçue = [de.sy], one syllable dé
- one çue; the ç is like s in see.
There is a liaison between réunion and a: you pronounce an extra n sound: réunion‿a été.