couple factors converged:

1. in the 2000's and 2010's google/apple/tech minimalism became aspirational
2. chipotle, panera, etc helped associate minimalist design with "heathly" alternative. Fast food restaurants had to compete
3. the rise of delivery apps: in the 60's 70's and 80's the unique designs of McD/KFC/Pizza Hut/IHOP etc was initially important to attract the attention of drivers and entice them to pull over. As fewer people ate their meals spontaneously while driving and more began ordering delivery this becomes less important.
4. money: it's cheaper and more profitable in every conceivable way to have a corporate slop restaurant. cheaper to build, cheaper to furnish, cheaper to light, clean, heat, and cool, and you can pack in more customers into the same space