Why is the Quran in Arabic?

Mostly because Arabic was the language of the people it was first revealed to. The Quran says Allah sent it in clear Arabic so that its first audience could understand it. Muslims keep the Arabic for worship because it preserves the exact words, and they treat translations as helpful explanations of the meaning, not as the Quran itself.

What the Quran says about its language

إِنَّآ أَنزَلْنَـٰهُ قُرْءَٰنًا عَرَبِيًّا لَّعَلَّكُمْ تَعْقِلُونَ "Indeed, We have sent it down as an Arabic Quran so that you may understand." Quran 12:2

The Quran also explains a general principle about every messenger:

وَمَآ أَرْسَلْنَا مِن رَّسُولٍ إِلَّا بِلِسَانِ قَوْمِهِۦ لِيُبَيِّنَ لَهُمْ "We have not sent a messenger except in the language of his people to clarify [the message] for them." Quran 14:4

The Prophet ﷺ was from Makkah, and his people spoke Arabic. Revealing the Quran in their language meant they could understand it directly. The Quran also notes that if it had been sent in a non-Arabic language, they would have complained (Quran 41:44).

Why the Arabic stays for worship

Exact wording: Muslims believe the words themselves are Allah's speech, so the Arabic is recited in prayer.
Preservation: Allah says He will protect the Reminder (Quran 15:9), and a single fixed text makes this easier to see in practice.
Unity: Muslims from every country pray with the same words, even if they speak different languages.
Depth of meaning: many Arabic words carry several shades of meaning that no single English word can hold.

Why translations are only interpretations

A translator has to choose between possible meanings, and different translators choose differently. That is why scholars call them "translations of the meanings" and why you may notice different wording in different editions. Reading a translation is still very valuable, because it is how most people begin to understand the message. See Is a Quran translation still the Quran? And to pick one, see Which English translation of the Quran should I read?

You do not need to be fluent in Arabic

Many Muslims around the world do not speak Arabic as their first language. They read in translation, listen to recitation and learn the Arabic step by step. If you are starting out, read for meaning first and add the Arabic gradually. See Can I read the Quran if I can't read Arabic?

A note for those who are curious

If you are not Muslim, you can read the Quran in English to understand its message and decide what you think. Many people find it helpful to read it alongside a short explanation of the background of the verses.

Key points

The Quran says it was revealed in Arabic so its first audience could understand it (Quran 12:2).
Every messenger was sent in the language of his people (Quran 14:4).
Arabic is kept for worship to preserve the exact words.
Translations explain the meaning but are not the Quran itself.

Tip: Read the translation first, then listen to the recitation while following the Arabic. Over time you will start to recognise common words.