It's to distance themselves from their past image. Bright vibrant colors and lots of goofy advertising make it look like a place for kids, or a place to get a treat. Subdued earthy tones and minimalist advertising makes it seem like a restaurant or a place to get a meal.

Could also be to avoid the bad image associated with them from documentaries like Super Size Me and Food Inc. A change in appearance says "well they're not EXACTLY the same as they were".

This just seems like a trend in modern architecture in general, at least when it comes to places of business. Nothing elaborate or colorful. Just a simple, relatively unadorned box with drab, muted colors. Even the logos have gotten more minimalist in design. Everything is just so… boring compared to 20th century designs. No character. No identity. Utterly soulless. I miss the bright colors, vivid neon lights, and varied designs from the 80s & 90s.