Permission to Marry a Woman from the People of the Book

Assalamu alaikum! 1.I wonder is there any known reason why Allah has permitted a muslim man to marry a woman from the people of the book? 2.Are there any restrictions , or can the muslim men just if they want to marry a christian or a jewish woman, do it just like that? It is mentioned so many times in the quran that is so important to be a good muslim and to die as a muslim. Allah compared a believing person and a non believing person as a living and a dead person. That the greatest sin is shirk and that they are the worst of people. I just don?t understand why it is allowed to marry someone committing this sin. Is there any known explination. I know that Allah is All-knowing and the most wise and He   knows best and wants only what is good for us.  But what is good with this kind of interfaith marriage? I don?t question His rulings but I just would like to understand why.the prophet Mohammed sawm adviced us to choose our friends , to be with those who brings us closer to Allah. But something that is so serious like marriage, how come you can live with someone that will not bring you close to Allah, actually the opposite. I already find it hard to have nonmuslim friends or non practising muslim friends. I have nonmuslim friends but I avoid spending much time with them because of  their bad habits and their bad way of life. I find that we have so little in comment. I don?t care why is not allowed for a muslim woman to marry a non muslim man, I don?t even wish that so it?s just fine. I accept this ruling as I do with His all other rulings because it?s from Allah and I must always obey him but I just wonder if there is any explanation why? Why is it a such a big distinction between for example a hindu and a christian or a jew.after all aren?t they all committing shirk? Allah said that all people that dies with shirk without repenting will abide in hell fire forever and only true, rigtheous muslims with the aid of the grace of Allah will enter paradise. 3.Or will Allah make a distinction between someone from the people of the book and someone beleiving in for ex. hinduism, buddhism or atheism etc. in the hereafter.