That is because Allah is the Truth, and that what they call upon other than Him is falsehood, and because Allah is the Most High, the Grand.
The Saheeh International Quran Translation
ذَٰلِكَ بِأَنَّ ٱللَّهَ هُوَ ٱلْحَقُّ وَأَنَّ مَا يَدْعُونَ مِن دُونِهِ ٱلْبَٰطِلُ وَأَنَّ ٱللَّهَ هُوَ ٱلْعَلِىُّ ٱلْكَبِيرُThat is because Allah is the Truth, and that what they call upon other than Him is falsehood, and because Allah is the Most High, the Grand.