WS:
Part of the reason is the absence of junk food - this stuff is usually wrapped and the wrapper discarded when the item is consumed. For example, in the Soviet times, the Russians used soda dispensers with an actual glass that was washed after each use (subject of derision by many yanks).
The main source of dirt in E. European cities is not trash but mud when it rains.
Another observation - throwing stuff away and general display of anti-social behavior is a uniquely Yank thing - if you did that in EE you could be admonished by passers by. Also, in the Soviet times they had an honor system in public transit - you put your fare in the box an tore a ticket out of a roll, although nothing prevented you from taking a ticket without paying. But as far as I know, most did pay.
I would also assume that these pics were taken when the stations were empty and presumably cleaned.
Wojtek