There are two main reasons for this, both equally important. Firstly, most people’s implicit conception of ‘art’ — picked up from school and along the way — tends to be narrow and blinkered, and fixed on the idea that ‘art’ is all about beauty and skilled craftsmanship — of the sort you find in state museums and national galleries — and not about anything else. This is an easy perspective to hold on to, and easy to apply, and it fits with a wider conception of what life is all about: striving to achieve special skills and abilities that are recognised by all. So when ‘art’ heads off in a completely different direction — as is the case with contemporary art — people are lost, and they go straight back to the old certainties, believing that ‘art is only about beauty and skill’.
The second reason contemporary art is widely disparaged is that the contemporary artworld itself — including all those thousands of artists, critics, gallerists, gallery goers and theorists– has a very poor understanding of its own principles, and so cannot make a convincing case for what it stands for. Contemporary art theory is a mess, and the general public is deeply sceptical, and perhaps with good reason.