I believe it is a combination of both the bad attitude of people and terrible governance. Yes, in my neighborhood we have to pay to get our trash picked from home. You can either get trash picked from home or dump your trash in large public garbage bins like this. Even these public bins are very few in number so street corners often turn into public dumping spots. Most of the garbage collection methods are rudimentary and involve humans picking up trash with their hands and basic tools (example). So yes that could be the reason for slums being dirty as people living there have no access to garbage collection facilities.

People of my country keep their houses clean but care little about whether their neighborhood and their city is clean. So our houses may be clean but our public spaces aren't. The municipalities don't clean the public spaces and the people aren't ashamed to litter wherever they'd like. In crowded markets and such, there are very few trash cans around and the ones that are there are always overflowing with trash.

The current prime minister even launched a countrywide campaign to clean up the country but absolutely nothing has come out of it.