Hello,
Why did Allah allow the Jews and Christians and others with revelations to corrupt their scriptures?
Assalam 'Alaikum Warahmatullahi Wabarakatuh
Since the man set foot on earth, they have been developing in all the means time by time. There came a time when the humankind had lived in caves of mountains etc. By time, stage by stage, step by step they were developing.
Since Allah is the creator of the Universe and everything in it, including Humans, He knows best about what is the perfect time to send this Shari'ah and that Shari'ah etc.
Allah has sent revelations to the mankind which fits to that time, to that people and when it is corrupted by the people of that time, Allah sends the revelation of next level, until He sends the final level which is the revelation send to Prophet Muhammad (pbuh).
If this Shari'ah was sent on the time of Adam for example, it will surely not fit on them at that time. In this Shari'ah, we cannot marry our own brother or sister, but at that time when humankind had just few people, marriage would never be possible if Allah sent this Shari'ah to them that the nearest marriage lawful relation is among cousins. So in the Shari'ah of Adam (pbuh), they were allowed to marry own brother or sister also.
For this purpose (to complete the laws until the final), the scholars says that Allah has allowed it to be corrupted so that then the people will have to follow the last and final revelation of Allah. Plus, those books are made Mansookh after the revelation of this Shari'ah.
Hope it helps my dear brother.
Wassalam 'Alaikum Warahmatullahi Wabarakatuh