According to W3Techs, as of the beginning of July 2013, 63% of all Joomla sites are running version 1.x. Of these, some 92% are running version 1.5. That works out to a rather large 58% of all Joomla sites running 1.5! The other 5% are mostly version 1.6 and 1.7. [Aside: if your site is one of those 5% please just upgrade now. It's not going to be that painful and you are a sitting duck for hackers. By "now" I mean stop reading this and go upgrade. Seriously.]
So why is the number so high? There are usually a long list of factors, and most of them are valid. Here are the ones I hear regularly: