﴿ وَقَالَ ٱلَّذِينَ كَفَرُوا۟ لَوْلَا نُزِّلَ عَلَيْهِ ٱلْقُرْءَانُ جُمْلَةًۭ وَٰحِدَةًۭ ۚ كَذَٰلِكَ لِنُثَبِّتَ بِهِۦ فُؤَادَكَ ۖ وَرَتَّلْنَٰهُ تَرْتِيلًۭا ﴾

“Now they who are bent on denying the truth are wont to ask. “Why has not the Qur’an been bestowed on him from on high in one single re­velation?” [it has been revealed] in this manner so that We might strengthen thy heart thereby - for We have so arranged its component parts that they form one consistent whole -”

Please check ayah 25:34 for complete tafsir.