Water Trees Grow. Insulation Weakens.
Soon after underground cables are installed, the wet environment causes water trees to grow. The growth of water trees reduces the dielectric strength of the insulation. The reduction of this dielectric strength can cause electrical/catastrophic failure of the cable.
Faced with this serious and growing problem in the 1980s, the prospect of addressing this problem on an industry-wide scale through cable replacement only would have been unsustainable, costing tens of billions of dollars and entailling a time frame that could not keep up with increasing failure rate.