There are a number of factors at play, but the most pertinent is possibly audience fatigue. When The Force Awakens debuted, it was the start of an ambitious plan from Disney and Lucasfilm. They wanted to release one Star Wars movie a year and though this was initially successful, with the J.J. Abrams-directed seventh entry shattering box office records left and right, the writing was on the wall shortly after.
The Last Jedi and Rogue One, while both incredibly successful, suffered from diminishing returns, with neither able to match the numbers that The Force Awakens posted. What’s more, the former quickly became the most divisive entry in the Star Wars canon.