In 1948 700000 Palestinians were outrooted of their homes and properties who took their properties And why arent they allowed to return ?

Those who ethnically cleansed them took their properties: Zionist Jews.
Israel prevents Palestinian refugees from returning because it claims they are a security threat. However, seeing as Israel is a settler colony built on stolen land, it did not have the population numbers to sustain itself. It could only be established by creating new demographic realities on the ground, these new realities necessitated that approximately 80% of the Palestinians in what is today considered Israel be ethnically cleansed to maintain a demographically stable Zionist ethnocracy. In short, Israel can only exist because millions of Palestinians are scattered refugees all over the world, simply because they are not Jewish.
Thousands of residents of Yarmouk Palestine refugee camp in Syria line up to receive humanitarian aid after UNRWA regains access to the camp in January 2014, providing a limited quantity of food and hygiene items at a distribution area inside the camp. © 2014 UNRWA Photo