well it was the 7th century Judaism and Christianity were very well established, and i would not use the word Plagarism ,more like assimilation.Its much easier to convert people with a religion they already have a grasp of ,than something completely alien.Then when anyone does start to make enquiries, you can say the older scripture is corrupted.
Response: But how would that make it much easier? If it's a clear forgery or assimilation then that would make a person not believe. How do you see it as the other way around? If someone today claimed prophethood and tried to convert me or other muslims with verses identical or similar to the qur'an, that would not make someone believe in them. Naturally, one would accuse them of forgery and call them a false prophet.