Short answer
Because Allah arranged it that way. The Quran came down in pieces over 23 years as events happened, and each time the Prophet ﷺ told the scribes where the new ayat belonged. Jibril reviewed the whole Quran with him in that arranged order every Ramadan. The order of ayat is fixed by revelation; most scholars say the order of surahs is too.
The order of ayat within each surah is tawqifi (set by revelation) by consensus. The majority, including the Hanafi scholars, hold that the order of surahs in the mushaf is also from the Prophet ﷺ, and that it is wajib to follow the order of ayat when reciting.
The four schools agree on this.
Explanation
The first ayat revealed were the opening of Surah al-Alaq (96:1 to 5), which is near the end of the mushaf. Al-Baqarah, at the front, came mostly in Madinah. This surprises many readers, but it was never hidden. Allah says He sent the Quran down in stages so that the Prophet ﷺ could recite it to people over a long period (17:106) and to strengthen his heart (25:32).
Two orders, one Quran. The order of revelation followed events: a question, a battle, a family case. The order of the mushaf is the order for reading and memorising, which Allah took upon Himself to collect (75:17). When new ayat came, the Prophet ﷺ told the scribes which surah they belonged to and where. In his final year, he told his daughter Fatimah that Jibril used to review the Quran with him once a year and had reviewed it twice that year.
The order of surahs. All scholars agree that the order of ayat is from revelation. Most say the order of the surahs is too. A minority said some surah order came from the Companions' judgement. Either way, the order we have is what the Companions agreed on, and no one may change it.
Can I read in revelation order? Yes, for study. Lists of the order of revelation exist, but they rely on reports and differ in places. In salah and in a normal khatm, follow the mushaf. See the history of the Quran and when the Quran was revealed.
The evidence
Quran, Surah Al-Israa 17:106
وَقُرْءَانًۭا فَرَقْنَٰهُ لِتَقْرَأَهُۥ عَلَى ٱلنَّاسِ عَلَىٰ مُكْثٍۢ وَنَزَّلْنَٰهُ تَنزِيلًۭا ﴿١٠٦﴾
And [it is] a Qur'an which We have separated [by intervals] that you might recite it to the people over a prolonged period. And We have sent it down progressively.
The Quran was revealed gradually.
Quran, Surah Al-Qiyaama 75:17-18
إِنَّ عَلَيْنَا جَمْعَهُۥ وَقُرْءَانَهُۥ ﴿١٧﴾ فَإِذَا قَرَأْنَٰهُ فَٱتَّبِعْ قُرْءَانَهُۥ ﴿١٨﴾
Indeed, upon Us is its collection [in your heart] and [to make possible] its recitation. So when We have recited it [through Gabriel], then follow its recitation.
Allah took upon Himself the collection and arrangement of the Quran.
Quran, Surah Al-Furqaan 25:32
وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةًۭ وَٰحِدَةًۭ ۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ ۖ وَرَتَّلْنَٰهُ تَرْتِيلًۭا ﴿٣٢﴾
And those who disbelieve say, "Why was the Qur'an not revealed to him all at once?" Thus [it is] that We may strengthen thereby your heart. And We have spaced it distinctly.
The reason for gradual revelation.
Sahih al-Bukhari 3624
Sahih... إن جبريل كان يعارضني القرآن كل سنة مرة وإنه عارضني العام مرتين ...
"Jibril used to review the Quran with me once every year, and this year he reviewed it with me twice."
The whole Quran was reviewed with the Prophet ﷺ in its arranged form.
Where the ruling is recorded
- al-Itqan (al-Suyuti), Chapter on the order of the ayat and surahs. Reports the consensus on the order of ayat and the views on the order of surahs.
- al-Burhan fi Ulum al-Quran (al-Zarkashi), Chapter on the arrangement of the surahs. Discusses whether the order of surahs is by revelation or by the Companions.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.