Shakespeare wrote many tragedies in his time. During this era, people all enjoyed tragedies, and flocked to theatres to hear them.
Romeo and Juliette was a fresh story to them. People loved romance, and they loved tragedy, so hearing about a story of two star crossed lovers was a fresh concept in the entertainment world. Everyone was curious to see how this played out. Which is why this was a very interesting play to see.
Shakespeare wrote plays based on the popular theme, tragedy in many of his works, which is why he was so successful. To this day people still honor this writer for his originality.