Final answer:
The primary reason it is difficult to understand the structure of the Milky Way is because interstellar dust obscures our view of distant stars. However, astronomers have made progress in mapping the galaxy using alternative methods such as radio waves. Additionally, our embedded position within the Milky Way presents a challenge in gaining a comprehensive view of its structure.
Step-by-step explanation:
The primary reason it is so difficult for us to understand the structure of the Milky Way is that interstellar dust obscures our view of stars at large distances in the disk at visible wavelengths. However, astronomers have been able to make progress in mapping the spiral structure of the Milky Way using alternative methods such as radio waves of 21-cm wavelength that pass through the dust and enable the detection of hydrogen atoms. Additionally, the fact that we are embedded within the Milky Way presents a challenge similar to trying to map New York City from Times Square instead of a helicopter, making it difficult to get a comprehensive view of the entire galaxy.