That's one thing for sure, but I attribute it most to the lack of dumpsters. I am not sure why they were banned, the trash cans supposedly because of terrorism, but the larger containers/dumpsters would make the city so much cleaner and better.
Tip: when there's a typhoon, restaurants close and there's no trash on the street, the rats invade nearby shops looking desperately for food:
I mean Tuesday: burnable waste, wed cardboard, Thurs clothes and Fri other, will tend to have trash in the designated areas, on each of those days.
Edit:spelling (Note:days and trash types differ a lot per area, but it's a good generalisation based off of my location).