Synthetic insulation to replace oil-impregnated paper in mains transmission cables?

Industry is well geared to the production of cables using oil-impreganted paper, and service experience with it, whilst not being perfect, has been extremely good. So why should a change be made from the well tried oil-impregnated paper to synthetic insulation? Essentially, the synthetic insulants offer lower dielectric loss, at least an order lower than oil-impregnated paper, relative permittivities ranging from 1 to 2.5, and extremely low moisture absorption