I think it might be down on how some people treat their local area, as someone else has said on the thread that buses in Barnsley were filthy - this could be down to how the local kids have been brought up not to look after the area in which they live, this habit is then passed onto the next generation who in turn will no doubt pass it onto the next and before you know it the local buses have become a mobile household waste site.
On a late night journey on the 363 that I took in the summer (a single deck no doubt) the near seating footwell was covered in bits of half eaten unwanted kebab or burger, mushed in chips and a discarded can that had been stamped on complete with a ruined copy of the Metro that has been stepped on - needless to say the bus had slowly turned into a bio-hazard.
I tend to find it varies a lot by operator. I find Transdev Keighley's buses are often much clearer than First Leeds, Bradford and Halifax buses - and I wouldn't put that down to differences in the type of people who live in those areas. Transdev do seem to take a bit more pride in the appearance of their buses - I don't know if that rubs off on people with them treating them a bit better.