The reason why Umar (RA) did this was he noticed as Islam expanded and adopted more languages the original fus7a Arabic was being gramatically changed. People started making mistakes that the original sahabah would never make people started using slang and other borrowed foreign words into their convos. Umar (RA) was a very pragmatic man and knew the hadiths being transmitted and over all Islamic studies would be affected so he temporarily banned it so the Arabic can maintain its original quality. His intentions were pure.