The reason is simple: tax-evation and the like, Swiss is well known for it around Europe. That it hasn't been dealt with is not strange since we live in the age of hidden plutocracy (trading outposts and the like exist in many countries for the rich to circumvent the law - this should put Brexit in a new light for some).
Things are slowly changing for the better as people start to realise what is the real problem within society (or rather outside society), but seing as what happened to the "Panama" leak where the concentraded information wasn't released and basically ICIJ sat on it for a year or so (the initial estimates during that time dwindles from over $62T to less than $30T, the official cause was accounting for the same money several times).