Why statistics is an art and not a science?

That "statistics is an art" is merely a common turn of a phrase. Statistics are in fact firmly based in mathematics which is widely regarded as one of (if not the) purest sciences.

However, the presentation and general (-y public) use of statistics on the other hand, is sometimes regarded as a largely deceptive practice which relies on subtle wording, technical definitions, and misdirection to relay a perceived meaning to the statistical results which is not necessarily there. That one set of numbers can relay two different meanings based on how they are presented is not the "art" of how you use the tools of statistics, the numbers and methods are absolute. The public interpretation of those statistics, which is more the realm of public speaking (pandering or Propaganda) is the so called "art" and relies on the general ignorance of statistics along with their interpretation and employ.

Short answer: It's not a science if you give those numbers to a liberal arts major (or anyone with an agenda).