You know.. there was once Reader Digest did a statistic study on which is the dirtiest toilet cubicle in an average toilet (please dun ask me which issue that was coz I only remeber the gist but not the details) and be surprised that the 2nd cubicle NEXT to the 1st one is the dirtiest. The cleanest is the middle one and tada! the 1st!
The pre-concept was the general mindset of most people thinks that the 1st cubicle is very dirty and so skipped it to use the next
The last cubicle has some truth... most like privacy when doing their business... so the last one is the best for that and therefore one of the more frequently used.