Those that don't either have 'issues' (ranging from anything from serious problems like alcoholism down to unusual opinions/unconventional lifestyles) which limit the number of possible partners they could have, or they 'punch above their weight' (ie, have unrealistic expectations of their partner in terms of looks, money, intelligence etc) and limit their possible partners in this way (I would consider myself to have suffered from both these symptoms).
It seems likely that if someones main priorities in life are watching Eastenders in their sportswear, communicating in text speak, or the classic "staying in and watching a DVD with a glass of wine" then the chances of being single are reduced. Their dating pool is sadly about 75% of the single UK population. You just need to vaguely fancy someone and it's a done deal. My mates (men/women) that struggle most finding a relationship are the ones that are the most articulate, interesting and stupid-funny left field. The dating pool (puddle) is tiny in comparison. It's not meeting someone that's the problem, it's meeting someone worth meeting. Then you're at the mercy of luck, looking for someone not tangibly described on paper.