People use DSA for the same reasons they use RC4, 3DES, and MD5: that's what the other party's 10 year old legacy platform that will never be updated supports.
But really it's about everyone just being human, and therefore crap at these things. Once security 'products' realise this, and cope well with people being rubbish at their jobs, then those products will make a lot of money... If only the people measuring ROI were also not rubbish at their jobs... ;)