Allah created one religion. Islam means 'submission to the will of god'. Why he chose not to protect the first scripture the same way he protected the last is like asking why Allah swt. allowed shaytan power over us, or why he chose to do something later rather than sooner. The answer is, he has infinite wisdom. Why did Allah swt. Make Prophet Adam (pbuh) capable of disobeying him? Surely we can never fully grasp the infinite wisdom behind Allah swts actions, but perhaps we can try to understand a glimpse of it. Perhaps if Allah swt. had not set the examples of past nations, people would have found it harder to believe. Maybe if the Quran was the only scripture, people would argue it has been altered. The Quran shows us many parables sometimes to show what not to do, and other times to show what the right path is. If the Quran was the only scripture, whose parables would Allah swt. Provide? The corruption of past religions was the peoples own doings, and the preservation of Islam was also the peoples own doings. Perhaps the past societies did not want to be guided. It's like asking why Allah chooses to guide some, and leave others. Maybe the people of the Prophets ummah (pbuhahf) were deemed worthy of guidance by Allah swt. Because they truly asked for it. There's many possibilities, but I think the main thing to understand would be that it's not because of Allah swt. That the first religions were not preserved. By that logic, all evil is also from Allah swt. What happened to those religions was a direct consequence of the actions taken by those people, and what happened with Islam is the reflection of Prophet Muhammads actions (pbuhahf), the support of his progeny, his rightful sahabas, and with that, the blessing of Allah swt. The right thing to note here would be that Allah swt. Gave humanity many chances, but muslims were the ones who took the advantage. Again, this question is very similar to 'Why didn't Allah swt. do xyz the first time'. Another way you could look at it is how the Christians dealt with prophet Jesus (pbuh) or how the Jews dealt with Prophet Moses (pbuhahf). Does Allah swt. the ranks of some over others? Yes, but it is due to the actions of these people, and the fact that they consistently ask for guidance and blessings from Allah swt. That they're able to reach that level, not the other way around.