One reason is because we have access to the text of the Qur'an, and therefore have the ability to judge its merits such as they are.
Given the problem in translation and the profound differences in some versions, you don't have access to the text of the Quran in Arabic unless you can read that language.
The best example I know of the problem of translation is 17:104. This example makes me very skeptical about those who rely on just one translation or even a couple to give opinions about what the Quran actually says. Because, depending on the translation, a non-Muslim could easily argue that this Sura proves we're living in the end times and that the state of Israel exists because of the will of God. Care to guess how many Muslims agree with this interpretation?
Palazzi (actually 17:100-104 giving some context) "Pharoah sought to scare them [the Israelites] out of the land [of Israel]: but We [Allah] drowned him [Pharoah] together with all who were with him. Then We [Allah] said to the Israelites: 'Dwell in this land [the Land of Israel]. When the promise of the hereafter [End of Days] comes to be fulfilled, We [Allah] shall assemble you [the Israelites] all together [in the Land of Israel]."
Khalifa And we said to the Children of Israel afterwards, "Go live in this land. When the final prophecy comes to pass, we will summon you all in one group."
Yusuf Ali And We said thereafter to the Children of Israel, "Dwell securely in the land (of promise)": but when the second of the warnings came to pass, We gathered you together in a mingled crowd.
Pickthal And We said unto the Children of Israel after him: Dwell in the land; but when the promise of the Hereafter cometh to pass We shall bring you as a crowd gathered out of various nations.
Shakir And We said to the Israelites after him: Dwell in the land: and when the promise of the next life shall come to pass, we will bring you both together in judgment.
Sher Ali And after him WE said to the Children of Israel, Dwell ye in the promised land; and when the time of the promise of the Latter Days comes, WE shall bring you together out of various peoples.'
"Progressive Muslims" And We said after him to the Children of Israel: "Dwell in the land, then, when the time of the second promise comes, We will bring you all together as a mixed crowd."
Al-Qurtub: And We said to the Children of Israel after him: "Dwell in the land, then, when the final and the last promise comes near [i.e. the Day of Resurrection or the descent of Christ ['Iesa (Jesus), son of Maryam (Mary) on the earth]. We shall bring you altogether as mixed crowd (gathered out of various nations).
Al-Shia.com We told the Israelites after this to settle in the land until Our second promise will come true. We would then gather them all together (on the Day of Judgment).