1- One primary reason is that if a woman is married to more than one man, then the father of the child generally won’t be known. If a woman has two husbands and she gets pregnant, normally it won’t be known which one is the father. And according to Islamic law it’s important that one’s father is known since many laws are based on one’s ancestry. Now you could argue that today we have DNA testing. First of all, Islam looks at situations holistically. DNA is a recent discovery. It was not available for over a thousand years after Islam. Secondly, even today it’s not available in many parts of the world, and it is costly. Many people cannot afford it. So it’s not a practical solution that works for most people, and when legislating a law, you have to look at the majority of people.
There are other reasons too (psychological and physiological).
2- There is no evidence that in paradise they won’t be allowed to marry more than one. So theoretically yes they can if that’s what they desire there.