Religion - Bible vs Koran

For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.
Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen. (Romans 1:18-25 ESV)

And if Allah had willed, He could have made you [of] one religion, but He causes to stray whom He wills and guides whom He wills. And you will surely be questioned about what you used to do. (Surah 16:93 Sahih International Version)

And if Allah willed, He could have made them [of] one religion, but He admits whom He wills into His mercy. And the wrongdoers have not any protector or helper. (Surah 42:8 Sahih International Version)

Why are there so many religions in the world?

According to the Bible, the cause is unbelief. People refused to believe in the truth, thus they believed in their own lies. Truth is one, but lies are infinitely many.

According to the Quran, Allah could have made all people with one religion, but he chose not to.